An improvement in the separator layer of an electrochemical heating element having two electrode layers, a separator layer of porous, absorbent material therebetween, and electrically conductive connector means extending through the electrode and separator layers. The improved device includes structural spacing means interposed between one of the electrode layers and at least a portion of the other electrode layer to limit compression of the separator layer material. In one embodiment, discrete mechanical spacers are placed immediately adjacent to the connector means. In another embodiment, a recessed surface portion is formed in one of the electrode layers and the adjacent separator layer has a mating portion on one side which is received into the recessed surface portion.BACKGROUND OF THE INVENTIONThis invention relates to electrochemical heat-generating elements, and, in particular, to improvements in uniformity and performance reproducibility in such elements.The prior art as taught by Kober in U.S. Pat. No. 3,774,589 described an electrochemical heater element having an anode layer and a cathode layer and a suitable porous, highly absorbent separator means situated therebetween, the electrode layers being connected one to another internally by electrically conductive short-circuiting members, such as staples and the like. Introduction of a suitable electrolyte into this construction initiates an electrochemical heat-producing reaction.It has been shown on theoretical grounds that this heater construction results in efficiencies of energy conversion, (that is, the conversion of the chemical energy inherent in the electrochemically active materials to thermal energy) approaching 100%. However, in practice, although the energy conversion reaction proceeds at an efficiency approaching 100%, reproducibility in the total heat output and output pattern is well below acceptable levels unless extreme care is taken during the manufacturing of these elements. Such extreme care and unreasonably close tolerances during manufacture are both time-consuming and costly. This problem applies to heating elements of the type disclosed in U.S. Pat. No. 3,774,589, and a new heat generating element which is disclosed in a concurrently filed, co-pending patent application of Frederick P. Kober entitled COMPLEX ELECTROCHEMICAL HEATING ELEMENT.Experimentation has shown that this less than acceptable level of reproducibility can be attributed to a non-reproducible compression of the absorbent separator material contained within the electrochemical heat producing element. Unless separator compression is eliminated or, at best, limited, the heat generating element absorbs varying quantities of activating fluid, that is, either water or an electrolyte solution, during activation. Furthermore, if the separator is compressed and held mechanically beyond a certain limit, the electrolyte which is absorbed is not able to distribute or wick uniformly throughout the heater structure. Hence, the heat generating reaction has a tendency to progress non-uniformly across the element with the result of less than maximum utilization of the available active materials.In short, this non-reproducible compression of the separator material occurring in normal assembly of the electrochemical elements results in a changing internal volume (from element to element) available for the absorption of electrolyte during activation. And, particularly if this separator compression is held beyond a certain limit, the absorbed electrolyte is not distributed uniformly throughout the heat-producing element.It has been demonstrated experimentally that, for simple electrochemical heating elements of the type disclosed in U.S. Pat. No. 3,774,589, using standardized anode layer, cathode layer and separator layer stock, and standard electrolyte concentrations, a minimum volume of electrolyte solution per square inch of heater element is required to insure efficient, reliable activation and to achieve maximum conversion of the active material to thermal energy. The minimum required volume of electrolyte has been determined to be on the order of 1.5cc per square inch of heater element for the standardized elements tested. Experience has shown, however, that, for a maximum rate of heat generation and maximum production of total heat, at least about 1.67cc of electrolyte solution per square inch of heater should be used. Substantial additional amounts of electrolyte solution above this minimum required volume do not significantly affect overall heater element performance. It has been found that an electrolyte volume in the range of about 1.67 to 2.4cc per square inch of heater element is effective. If an electrolyte volume much above about 2.4cc per square inch were used, the overall rate of heat generation would be diminished due to the larger mass of internal liquid to be heated.These minimum volumes of electrolyte are all above that required stoichiometrically to sustain the electrochemical heat-generating reaction. But such minimum volumes of electrolyte appear to be necessary: (1) to assure uniform wetting and distribution of the liquid throughout the heater structure, and (2) to provide sufficient electrolyte solution for the hydrolysis reaction(s) which accompany and probably compete with the electrochemical reaction. Regardless of the exact reasons, however, the heat-generating structure must be capable of absorbing at least a minimum required volume of electrolyte for it to function efficiently and reproducibly.In an attempt to improve the electrolyte capacity and increase the ease of reproducing performance characteristics of electrochemical heating elements of the type disclosed in U.S. Pat. No. 3,774,589, one might consider using a substantially thicker layer of separator material or a multi-layered separator. However the use of a thicker or multi-layered separator results in an increase in internal resistance of the heater element, causing a decrease in the rate of heat generation. Further, the use of a thicker or multi-layered separator does not substantially reduce or eliminate the problem of separator compression.Thus, there has been a need for an electrochemical heat-generating element having good reproducibility of performance and having substantial capacity for an electrolyte solution in its separator layer.BRIEF SUMMARY OF THE INVENTIONThe present invention is an improvement in electrochemical heat-generating elements of the type having at least two electrode layers, including an anode of electrochemically active, electrically conductive, oxidizable material and a cathode of electrochemically active, nonmetallic, reducible material, a separator layer of porous, absorbent material between the electrode layers, and electrically conductive connector means extending through the electrode and separator layers. The improvement relates to a structural spacing means interposed between one of the electrode layers and at least a portion of the other electrode layer whereby to limit compression of the separator layer.In one embodiment, the structural spacing means includes discrete mechanical spacers placed immediately adjacent to the connector means between the electrode layers. Also disclosed are variations of this embodiment. In another principal and highly preferred embodiment, the structural spacing means includes one of the electrode layers being formed to define a recessed surface portion and the adjacent separator layer having a mating (or "wick") portion formed on one side and received into the recessed surface portion of the electrode layer. Variations of this embodiment are also disclosed and claimed.A substantial improvement in reproducibility of performance characteristics and elimination of undue separator compression are obtained by using the aforementioned discrete mechanical spacers. Such spacers may take the form of small planar pads which are situated between the separator material and one or both of the electrodes and pierced through by the electrically conductive connector means such as staples. The area of such pads in comparison with the area of the heating element is negligible. However, the interposition of such pads within the heating element structure reduces the pressure of the electrode layers on nearly all of the surface of the separator layer by applying the pressure through the spacers. Other sorts of mechanical spacers, including small metal eyelets and a great variety of other configurations of metallic or electrically inert materials, can be used to advantage. By using such mechanical spacers, much of the separator compression is eliminated and the retained electrolyte volume can be controlled within much closer tolerances. In this way, heat output can be maximized with excellent reproducibility from heater to heater.While the use of such mechanical spacers has proven effective, there may be certain difficulties in properly and reproducibly placing such spacers. Accordingly, an important alternate embodiment of this invention is the inventive "recessed" structure described and claimed herein. In such embodiment, at least one of the electrode layers is formed to define a recessed surface portion which may be centrally located on the surface of such electrode layer. Such recessed surface is preferably formed in the cathode layer. The adjacent separator layer has an additional built-up portion, either added to or formed with the basic portion of the separator layer, which mates with the recessed surface portion of the electrode layer. The electrically conductive connector means, such as staples, extend through the heating element in surface areas away from the recessed and mating portions.By virtue of this configuration, the mating portion of the separator layer can function as a wick for the separator layer, assuring that sufficient electrolyte solution is available. The recessed portion and mating portion (or wick) may be in a variety of shapes and sizes. Preferably the wick is within the range of about one-eighth to one-half of the width of the heat-generating element and most preferably on the order of about one-third the width of the element. However, it may be appreciated that a variety of sizes and shapes are operable and in some cases a multiplicity of wicks and a multiplicity of recessed portions would be acceptable.As already indicated, it is preferred that the recessed surface portion be formed in the cathode layer. However, the anode layer may be used for this purpose as well, although the performance which has been achieved in such structures has not been as dramatic as that derived from the preferred arrangement. In simple electrochemical heating elements, having one anode layer, one cathode layer and a separator layer, both the anode and cathode can be formed with a recessed portion. However there appears to be little if any additional advantage achieved with such a configuration. In complex elements having two cathode layers, two separator layers, and a center anode layer in a sandwich-like structure, each cathode layer may be formed to have a recessed surface portion and the adjacent separator layers may be formed with appropriate mating portions to provide a wicking structure for each separator.The highly preferred embodiments having the wick structure provide excellent reproducibility in performance from element to element. Furthermore, cost advantages are realized in production since special manufacturing procedures and extremely close tolerances may be eliminated. Even if the basic separator material, around the mating or wick portion, becomes compressed during assembly, the uncompressed electrolyte resevoir provided by the wicking structure is available to absorb the necessary quantity of electrolyte and distribute such electrolyte uniformly throughout the heater.The benefits and advantages derived from this invention, and, in particular, from the most preferred embodiment including the wicking structure, are especially important for electrochemical heating elements requiring a high rate of heat generation, sustained heat output over an extended period of time (for example, 20 minutes or longer), uniform heat generation per unit area of heater element, and/or rapid reliable activation. Elements designed for such performance have a generally higher sensitivity to variations caused by normal manufacturing procedures.Comparative data illustrating the advantage of reproducibility provided by this invention will be given hereafter.OBJECTS OF THE INVENTIONOne object of this invention is to overcome the aforementioned problems and shortcomings in electrochemical heating elements of the prior art.Another object of this invention is to provide electrochemical heating elements which are substantially uniform in performance.Another object of this invention is to provide an electrochemical heating element which may be manufactured with normal manufacturing procedures without concern for extremely close tolerances.Yet another object of this invention is to provide an electrochemical heating element in which there is a high utilization of electrochemically active materials.These and other important objects of the invention will be apparent from the following description of preferred embodiments and the discussion relating thereto.