A CASE AND A METHOD OF MANUFACTURING SUCH A CASE The present invention relates to a piece of baggage or "case" and a method of manufacturing such a case. More particularly, the invention relates to a case comprising: a rigid shell essentially comprising a bottom wall extending substantially in a plane, and a side wall extending around the entire periphery of the bottom wall and away from the plane of the bottom wall, the bottom wall and the side wall thus defining an internal space with an open face; and a rigid lid closing the open face when the case is closed. Document FR 2 804 294 describes an example of such a case whose volume is formed by two shells connected together via one edge of each shell, the edges of the shells extending in wide curves that are complementary in pairs. The case described in that document is intended to form a rigid container that is extremely resistant to mechanical stress such as stress generated by the container being crushed, dropped, or subjected to impacts, etc. In alternative manner, the invention provides a case which, in addition to having some of the above-mentioned characteristics, is characterized by the fact that the shell is provided with a rim at the periphery of the open face, the shell thus having eight corners between which the internal volume occupies substantially the entire volume of the case. In this document, the word "corner" can designate a true corner, but more generally a corner that would be at the intersection of the main faces of the case. Consequently, a "corner" can be defined as a trihedral constituted with three main faces of the case. By means of these provisions, the case of the invention has a volume essentially defined by a single one-piece shell. All of the corners and edges are on said one-piece shell. Thus, when the case of the invention is subjected to stresses generally corresponding to forces that are larger on the non-plane surfaces such as the edges and the corners, it is the entire one-piece structure that takes up said forces. A case is thus obtained that is stronger than prior art cases such as a case made up of several parts, notably made up of two half-shells, and in which stresses tend to cause each part to fret relative to the other, thereby leading to said parts coming apart or even separating completely, resulting in the destruction of the case, or at least resulting in the belongings contained in the case being lost. In certain embodiments of the invention, it is possible optionally to use one or more of the following provisions: - the shell is made of a composite material; - the lid is made of a composite material; and - the composite material is made up of carbon fibers impregnated with a synthetic resin. In another aspect, the invention provides a method of manufacturing a case implementing a mold for making a shell for a case, the mold being made up of at least one portion defining an inside surface, said method comprising operations consisting in: lining the molding surface of the mold at least in part with at least one piece of a component material of the case; pressing said at least one piece on the molding surface with a flexible pouch on which is exerted a pressure. In certain implementations of the method, it is possible optionally to use one or more of the following provisions : - each piece of the component material of the case is made of a composite material; - the composite material is made up of carbon fibers impregnated with a synthetic resin; - the mold is made up of a plurality of portions, and the portions are assembled together before the inside surface of the mold is lined with at least one piece of component material of the case; - the flexible pouch is inflated inside the mold in order to remove as much air as possible between the mold and said piece; - the mold in which at least one piece of the component material of the case is deposited is placed in a flexible pouch which is then evacuated so as to remove as much air as possible between the mold and said piece; - a plurality of portions of the mold are disassembled in order to extract the shell; - the mold in which the component material of the case is deposited is subjected for in the range 1 hour to 2 hours to a pressure lying in the range 5 bars to 10 bars and to a temperature lying in the range 1000C to 1700C; and - inserts are fixed to the shell, after it has been unmolded, by adhesive bonding and curing, at a temperature lying in the range 5O0C to 1000C for in the range 1 hour to 2 hours. Other aspects, objects, and advantages of the invention will appear on reading the following description of one of its embodiments. The invention will also be better understood with reference to the drawings, in which: Figure 1 is a diagrammatic perspective view showing an example of a case of the present invention; and Figure 2 is a diagrammatic view showing an example of implementation of the method of manufacturing a case such as the case shown in Figure 1. In the various figures, like references designate elements that are identical or similar. An embodiment of a case of the invention is shown in Figure 1. In this embodiment, the case 1 comprises a shell 2 and a lid 4. The lid 4 and the shell 2 are made of a composite material such as fibers impregnated with synthetic resin. For example, they may be carbon fibers impregnated with epoxy resin. The lid 4 is made up of a cover 6 and of a backing cover 8. The cover 6 is designed to form a portion of the outside surface of the case 1, while the backing cover 8 is designed to form one of the inside faces of the case 1 when the lid 4 is closed. The shell 2 is essentially in the form of a one-piece box. The one- piece box comprises a bottom wall 10 and a side wall 12. The bottom wall 10 extends substantially in one horizontal plane when the case 1 is open in a position adapted to stowing belongings in it. The side wall 12 extends substantially perpendicularly to the bottom wall 10, and over the entire periphery thereof. It is made up of four faces 12a, 12b, 12c, and 12d. Opposite from the bottom wall 10, the side wall 12 is extended by a rim 14 in a plane substantially parallel to the bottom wall. The shell 2 thus defines an internal space 15 with an open face corresponding to the surface left vacant inside the rim 14. The shell 2 thus has eight corners 3 and twelve main edges between which the internal space 15 occupies substantially the entire volume of the case 1. The rim 14 constitutes reinforcement that stiffens the case and is effective in increasing the strength of the side wall 12. A single hinge 16 is fixed firstly to the side wall 12 and secondly to the lid 4. In order to improve the sealing of the case 1, a gasket 20 is fixed to the lid 4. The gasket 20 is designed to co-operate with the rim 14. At four of its main edges, the shell 2 is provided with grooves serving to receive protective edging strips 22, e.g. made of wood. The shell 2 is also provided with reinforcement 24 underlying a handle 26. The shell is also provided with notches 28 in which small wheels are fixed 30. Making the complex shape of the shell 2, in particular at the rim 14, at the grooves for the edging strips 22, at the reinforcement 24 for the handle 26, and at the notches 28 for the wheels 30, etc., requires a special manufacturing method. In certain models of case not described herein, a channel can even be provided in the rim 14. An implementation of the method of the invention is described below with reference to Figure 2. In this implementation of the method, pieces 31 are cut out to pre-established patterns from a sheet of carbon fibers pre-impregnated with epoxy resin. Some of the pieces 31 are to serve to form the main faces of the case. Deposited in one or more thicknesses, they can then be referred to as "skins" 312. Others of the pieces 31 serve to form pieces of reinforcement 311 which are interposed between or deposited on the skins 312, at the edges, at the points at which the hinge 16 is fixed, or at points at which other elements are fixed, etc. A first set or "kit" of pieces 31 serving for manufacturing a case are cut out manually by an experienced operator. The pieces of the first kit are than digitized, in order to optimize use of the raw material, and in order to enable the various subsequent kits to be cut out automatically. Once the pieces 31 in each kit have been cut out, the kit is enclosed in a plastics film before being used in the next few hours . The pieces 31 in each kit are then deposited in a layout that is precisely determined in a mold 34 comprising various mold portions 32 that are assembled together to form it. The number of the portions 32 is, for example, thirteen for the mold 34 of the shell 2 (Figure 2a) . For example, the mold is made of carbon or of metal. For example, first and second skins 312, and the reinforcing pieces 311 are disposed in the mold. The pieces 31 are intrinsically sticky, and they can thus line the inside of the mold 34. A flexible pouch 36 is inflated inside the mold 34, in order to press the pieces 31 against the portion of the inside surface of the mold that it covers (Figure 2b) . Depending on the geometrical shape of the piece to be obtained, the flexible pouch 36 can be placed inside a mold and then inflated, or else a mold lined with pieces 31 is placed in the flexible pouch which is evacuated. For certain geometrical shapes, it is also possible to combine inflating a flexible pouch to press pieces 31 against the surfaces of a mold, into recessed portions, while another flexible pouch is evacuated to press other zones of the pieces 31 against projecting surfaces of the mold. The mold 34 is then placed in an oven for 1 hour 30, at a pressure of 6 bars and at a temperature of 1300C, for a polymerization cycle. When a plurality of models of case exist in a range, all of the models are preferably placed under the same conditions, and undergo the same polymerization cycle. After polymerization, the pieces are unmolded manually, and then inspected (Figure 2c) . Any burs and other undesired portions are cut off using a water jet. The cover 6 and the backing cover 8 of the lid 4 are formed using the same method as the method used for the shell 2. They are then assembled together by adhesive bonding, at a temperature lower than the temperature of the polymerization cycle, after they have been placed in a suitable jig. Inserts 38, such as nuts for fixing a hinge, or a lock, etc. are then fixed to the lid 4 and/or to the shell 2, by adhesive bonding, e.g. with resin, and curing. The various portions of the case are then assembled together conventionally (Figure 2d) . The case 1 obtained using the method of the invention is particularly strong. The following table summarizes tests preformed to pre-established standards that are well known to the person skilled in the art on a case of the present invention. The numbering 1, 2, 3, 4, 5, and 6 in the above table, for the faces of the case, corresponds to a standardized numbering system. The corresponding faces are respectively, with the references used above, the faces 12a, top of the lid 4, 12c, outside face of the bottom wall 10, 12b, and 12d. The case tested withstood all of these tests without being significantly damaged and its integrity has been preserved.