DEVICE FOR HEATING A MOLD The invention relates to a device for heating a mold. The invention is particularly but not exclusively suitable for making a large mold designed for 5 molding composite materials using a method known an as out-of-autoclave method. The molding of a composite material with a thermoplastic or thermosetting matrix from prepreg plies requires an operation to cure/consolidate the preform made up of the layered structure of said plies. That curing/consolidating operation is generally carried out in an autoclave, 10 that is to say in a large fully closed enclosure with a heating system and a pressurizing system, where the layered structure is bagged and a vacuum is applied to it. In particular, such an autoclave makes it possible to obtain uniform temperature in the layered structure during the curing/consolidating operation. An autoclave is a piece of production equipment with a high cost, 15 and, particularly when it is adapted for making large parts, is a unique resource within a production system, the availability of which determines production management. The price of an autoclave is exponentially proportional with its diameter and with the cost of the door to the inside of the autoclave, which must be sealed when it is subjected to pressure and a 20 large temperature difference between its inner and outer faces. Thus, the more an autoclave is capable of working with large parts at high temperature, the higher its cost. In order to avoid the constraints imposed by the availability and cost of such a piece of equipment, particularly for working with composite materials with organic matrices, methods known as 25 out-of-autoclave methods are used particularly for curing or consolidating parts made of such materials. These out-of-autoclave methods use independently heated molds or molds that can be placed in a stove and means for increasing the pressure inside the mold. Typically, out-of-autoclave methods for working with a composite material 30 with fiber reinforcement in thermosetting resin are methods that use resin injection, by transfer or infusion, where vacuum is applied to the mold cavity, 2 wo 2015/155369 PCT/EP2015/057993 before the process or otherwise. The most conventional methods initially involve laying up plies of dry fibers, inserted in the closed cavity of a mold, and liquid resin is injected under pressure in said cavity. The mold used for these methods is a rigid closed mold that defines a cavity between two 5 parts, which mold is designed to withstand the pressure inside said cavity and the corresponding closing force. The preform made of dry fibers is located in the cavity between the molding faces of the two rigid parts. That is true, for example of the RTM (Resin Transfer Molding) method. These out-of-autoclave methods include vacuum-assisted methods such as 10 the LRIVAP or Liquid Resin Injection Vacuum Assisted Process method or the VARTM or Vacuum Assisted Resin Transfer Molding method, which use tooling known as soft tooling, in which a layered structure of dry fibers is laid up on a moiding face of said tooling. Said layered structure is bagged and a vacuum is applied to it before the resin is injected. 15 When these out-of-autoclave methods are used according to the prior art for large parts such as, as non-limitative examples, parts of the wing or fuselage of an aircraft or a wind turbine blade, uniformly heating the cavity of the mold or the matrix in which the layered structure is located is tricky. In the case of a mold with two faces defining a closed cavity, the mass of the 20 mold is large and requires a lot of energy. The use of a mold with a single molding face makes it possible to reduce the mass of the molding device, but only to some extent, because the reduction of thermal inertia becomes a handicap for obtaining uniform temperature. Further, it is difficult to insert a device for heating by fluid circulation or by electrical resistance into a 25 lightweight mold without increasing the sections of the mold. The lack of uniformity of the temperature is also liable to produce distortions in the shape of the mold, in addition to its influence on the flow of resin. Thus, these methods, particularly when used for large parts, are not suitable for application with an independent mold, and are commonly applied in a stove. 30 While such stoves represent a smaller investment than that required for an autoclave, they do however pose the problem of their availability and require 3 wo 2015/155369 PCT/EP2015/057993 heating a fully closed volume that is larger than that made up of the cavity of the mold or the layered structure. The document EP1 894 442 describes a method for heating a molding surface using inductors inserted in cavities, grooves or bores, machined in a 5 mold. The thickness of the material located between said cavities and the molding surface is used to make the temperature of the molding surface uniform during heating resulting from the circulation of alternating current in said inductors. Thus, this type of tooling, which is satisfactory with mediumsized parts such as the hoods of automotive vehicles, requires a carcass 10 that is relatively massive and turns out to be expensive with very large parts such as those for which the invention is intended. The document EP 1 728 411/US 7 679 036 describes a device and a method for processing material contained in a cavity, particularly a sealed cavity, comprising two half molds that are electrically conductive placed 15 opposite each other, where the opposite faces of the two half molds which demarcate the cavity are electrically insulated from each other, and are made of magnetic material. The two half molds are surrounded by the coils of an induction circuit. The gap created between the two half molds makes it possible to make induced currents circulate on the faces of the cavity and 20 thus obtain heating focused on those faces without integrating heating means in the half molds such as an electrical resistor, a fluid (vapor or oil) circulation circuit or inductors. In that example of the prior art, the induction circuit is configured in two separable parts, joined to each of the half molds and connected mechanically and electrically when said half molds are 25 brought closer in order to close the cavity. Thus, this device of the prior art is particularly designed to be used in combination with a means to open and close the mold, such as a press. The invention aims to remedy the drawbacks of the prior art and therefore relates to a device for heating a molding face, particularly a large one, which device comprises: 30 a. a metal sheet comprising a ferromagnetic layer and comprising a part formed into a shape that defines said molding face and a forming plane; 4 wo 2015/155369 PCT/EP2015/057993 b. a base and means to support said metal sheet on said base; c. means for the induction heating of said metal sheet. Thus, the use of induction heating means makes it possible to achieve the uniform, fast and controlled heating of a fine metal sheet making up the 5 molding face in a method known as an out-of-autoclave method. The invention can be implemented advantageously in the embodiments described below, which may be considered individually or in any technically operative combination. Advantageously, the base is made of non-metallic material such as concrete 10 or a ceramic. Thus, a rigid base is made in an economical manner, as the mass of said base is not heated. Advantageously, the metal sheet comprises a ferromagnetic layer made of an aHoy 1X)1Tlprising iron and nickel, of the lnvar type. Thus, the device according to the invention is particularly suited for processing composite 15 materials with organic matrices, at a temperature close to the Curie temperature of lnvar, with a coefficient of thermal expansion that is low, such as composites with epoxide matrices and reinforced with continuous carbon fibers. Advantageously, the metal sheet comprises a ferromagnetic layer made of 20 nickel (Ni). Thus, the device is suitable for materials that require a higher processing temperature, particularly composites with high-performance thermoplastic matrices. Further, that material is easily processed by additive machining techniques. In one exemplary embodiment, the device according to the invention 25 comprises: d. bagging means that demarcate, with the molding face, a sealed cavity adapted to contain a fibrous preform; e. means to apply vacuum to the volume located between the molding face and the bagging means. 30 Thus, according to this embodiment, the device according to the invention is suitable for the application of an out-of-autoclave method to a molding face. 5 wo 2015/155369 PCT /EP2015/05 7993 More particularly, the device according to the invention is suited to the consolidation in shape of a stratified composite part with a thermoplastic matrix and the implementation of methods designed for composites with thermosetting matrices such as the LRIVAP method or the VARTM method. 5 To that end, the device according to this exemplary embodiment of the invention advantageously comprises: f. means to inject resin in the cavity demarcated by the molding face. Advantageously, the device according to the invention comprises: g. a circuit for the circulation of cooling fluid in contact with the metal sheet. 10 Thus the forced rapid cooling of the molding sheet, combined with the rapid heating capability of the induction means, reduces the cycle time. In a particular embodiment, particularly suitable for the curing/consolidation of thick parts, the bagging means comprise a bladder comprising heating means. Thus, the heating applied to the other face of the part makes it 15 possible to reduce the temperature gradients in the thickness of the part during the curing/consolidation cycle. In a first embodiment of the device according to the invention, the molding face of the metal sheet is ferromagnetic and said device comprises: h. an enclosure with a volume demarcated by the coils of an induction 20 circuit; i. means to load, unload and hold the metal sheet and its support inside the coils of the induction circuit. Thus, the enclosure of the device according to the invention is not necessarily closed and does not need to be thermally insulated. The flow of 25 current in the coils of the induction circuit leads to the direct heating of the molding face of the metal sheet, as a result of the circulation of induced currents on that face, with no transfer of heat by conduction/convection between the air of the enclosure and the mold. The heated volume is extremely small and uniform heating is obtained by controlling the flow of 30 induced currents in it. 6 wo 2015/155369 PCT/EP2015/057993 In a particular alternative of this first embodiment, bagging is made of electrically insulating material and said device comprises: j. an electrically insulating shim on the perimeter of the metal sheet; k. a counterpart made of electrically conductive material and adapted to 5 apply pressure on a preform in the cavity located between the bagging and the molding surface when a vacuum is applied to said cavity, and comprising a face opposite the molding face in order to create a gap between said molding face and the face opposite the counterpart. Thus, the counterpart creates a gap and further improves the efficiency and 10 control of the heating of the molding surface, particularly by controlling the thickness of the gap. This alternative embodiment makes it possible to use the counterpart as a caul plate, and the insulating shims as packing shims to motr