The assistance to the effort exoskeletons are mechanical structures that duplicate the human skeletal structure that improve the physical capabilities of the human body. There are different types of support to the effort exoskeletons, dependent tasks by the user. Some structures exoskeleton include actuators to assist the user in his movements, and a battery to power the electric power actuators necessary for their operation. Generally, actuators arranged in the lower parts of the exoskeleton (including parallel joints of the lower limbs) tend to consume more power than the actuators arranged in the upper parts of the exoskeleton (including parallel joints senior members). Indeed, the actuators disposed in the lower part must bear all the weight situated above, the weight of the user and the weight of the structure. This is the case in particular hip or knee actuators designed to assist the flexion or extension of the lower limbs. These actuators are particularly consumers of electrical energy, particularly when the user performs large amplitude movements. SUMMARY OF THE INVENTION An object of the invention is to provide an exoskeleton structure including actuators to effectively assist the user in his movements, while limiting the consumption of electric energy of the actuators. This is achieved in the context of the present invention with an exoskeleton structure for assistance to the effort of a user, comprising: - a first own module to be fixed on a first body part of a user, - a second own module to be fixed to a second body part of the user, the second module being connected to the first module by means of a joint allowing rotation of the second module to the first module, - an actuator comprising a stator and a rotor own to be rotated relative to the stator to move the second rotation module to the first module, and - a resilient return element arranged to: in a first angular range of movement of the rotor relative to the stator, exert no biasing force on the rotor, and in a second angular range of movement of the rotor relative to the stator, engage a restoring force tending to oppose a rotation of the rotor relative to the stator in a first direction of rotation and to assist rotation of the rotor relative to the stator in a second rotational direction opposite the first direction of rotation. The proposed exoskeleton structure combines an active support (through the actuator) and a passive assistance (via the elastic return element), depending on the type of movement performed by the user. In the first angular range of movement of the rotor of the actuator, the user benefits from an active support, while in the second angular range of movement of the rotor of the actuator, the user benefits in whole or in supplement a passive audience. In this way, the power delivered by the actuator is limited in the second angular range of movement. For example, in the case of a hip joint or knee, the first angular range of movement can be defined as an angular range in which the rotor is located when the user walks or runs, while the second angular range of movement can be defined as an angular range in which the rotor is located when the user is sitting or crouching. In this way, the exoskeleton structure allows fine control of the assistance provided by the actuator in the walking or running phases (Phases in which the electric power consumption is the lowest), while providing assistance effective in cases of full flexion of the hip or knee. The proposed exoskeleton structure can also have the following characteristics: - the first module is a basic module comprising a clean hip belt to surround the waist of the wearer, the second module is a clean hip module to be fixed on the thigh of the user and the joint is a joint hip, - in this case, the first angular range may correspond to a movement in flexion or extension of the thigh of between +60 degrees and -15 degrees relative to a frontal plane of the user, and the second angular range may correspond to movement in flexion or extension of the thigh greater than 60 degrees relative to the frontal plane, - the first module is a clean hip module to be fixed on the thigh of the user, the second module is a clean knee module to be secured to the calf of the user, and the joint is a knee joint , - in this case, the first angular range may correspond to a movement in flexion or extension of the calf between +15 degrees and -60 degrees relative to a frontal plane of the user, and the second angular range may correspond to a movement in flexion or extension of the lower calf at -60 degrees relative to the frontal plane, - the first angular range of movement is defined as an angular range in which the rotor is located when the user walks or runs, and the second angular range of movement (<¾) is defined as an angular range in which the rotor is when the user is sitting or squatting, - the structure comprises a stopper fixedly mounted on the rotor, the lug being adapted to bias the resilient member in compression when the rotor is in the second angular range, - when the rotor is in the second angular range, the elastic element exerts on the rotor a restoring force which tends to resist bending of the second body part relative to the first body part, and attend an extension of the second body part relative to the first body part, - the elastic return element comprises a prestressed spring, - the structure comprises a stop against which the elastic element rests and whose position relative to the stator is adjustable by means of screws. PRESENTATION OF DRAWINGS Other characteristics and advantages will emerge from the following description which is purely illustrative and non-limiting and should be read with reference to the appended figures, in which: - Figure 1 shows schematically, in front view, a user equipped with an exoskeleton structure according to a possible embodiment of the invention, - Figures 2 and 3 show schematically, in back view and in side view, the user equipped with the exoskeletal structure, according to a first possible configuration of the invention, - Figures 4 and 5 show schematically, in back view and in side view, the user equipped with the exoskeletal structure, according to a second possible configuration of the invention, - Figures 6 and 7 schematically show, in rear view and in side view, the user equipped with the exoskeletal structure, according to a third possible configuration of the invention, - Figures 8A and 8B schematically illustrate a hip joint connecting a hip module to the basic module, - Figure 9 shows schematically a fastening device for attaching the hip module to the basic module, - Figures 10A and 10B show schematically the attachment device in the unlocked configuration and respectively locked configuration, - Figure 1 1 shows schematically the lower modules of the exoskeletal structure, - Figures 12A-12E schematically illustrate a boot equipped with a foot module during different from the user walks phases, - Figure 13 shows schematically an enlarged view of the upper modules of the exoskeletal structure, - Figure 14 shows schematically a spine segment forming part of the back of the module, - Figure 15 schematically shows a vertebra of the spine member segment, - Figures 16 to 18 show schematically a shoulder module, - Figure 19 shows schematically a fastening device for attaching the shoulder module to bend module, - Figures 20 and 21 schematically show a backpack carrier module, - Figure 22 shows schematically a fastening device for attaching the support module backpack hip each module. DETAILED DESCRIPTION OF AN EMBODIMENT modular structure In Figures 1 to 7, the exoskeletal structure shown comprises a base module 1, a back module 2, two shoulder modules 3, two elbow modules 4, two hip modules 5, two knee modules 6 , two foot modules 7 and a bag support module 14 back. The exoskeleton shown in these figures structure can be used in different configurations in order to obtain different exoskeletons adapted to different uses. In a first configuration illustrated in Figures 1 to 3, the exoskeleton is formed by the assembly of the basic module 1, module 2 back on both shoulder modules 3, of the two elbow modules 4, the two hip modules 5, the two knee modules 6 and 7 foot two modules. In a second configuration illustrated in Figures 4 and 5, the exoskeleton is formed by the assembly of the basic module 1, module 2 back on both shoulder modules 3 and two elbow module 4 only. In a third configuration illustrated in Figures 6 and 7, the exoskeleton is formed by the assembly of the basic module 1, two hip modules 5, the two knee modules 6, the two leg modules 7, and bag support module back 14 only. The three examples of configurations illustrated in these figures are obtained from three different assemblies of the modular structure of exoskeleton. However, other configurations are of course possible. In these various configurations, the exoskeleton is formed from one or more modules assembled together. As illustrated in Figures 1 to 3, the basic module 1 comprises a waist belt 1 one is able to surround the bottom of the wearer's trunk. Lumbar buckle 1 1 is arranged around the waist of the user, resting on the hips of the user. The basic module 1 also includes a first battery 12 for powering the various actuators of the electric power structure, and a control unit 13 programmed to control the various actuators. The first battery 12 and the control unit 13 are fixed to the waist belt 1 January. The back module 2 is adapted to be fixed to the upper body of the user, above the basic module 1, along the back of the user. bend 4 of the modules are adapted to be secured on the user's arm, respectively in the right arm and left arm. Each shoulder module 5 is adapted to connect the back of module 2 to a respective elbow module 4. The back module 2, the shoulder and the elbow 3 modules 4 modules form a set of upper modules whose function is to assist the user in much it exercises with his upper body, e.g. when performing repetitive tasks with his upper body. 5 hip modules are adapted to be secured over the thighs of the user, respectively in the right thigh and the left thigh of the user. The knee modules 6 are adapted to be secured to the calf of the user, respectively in the calf of the right leg and calf of the left leg of the user. 7 foot modules are adapted to be secured on the user's feet, respectively, right foot and left foot. Hip modules 5, the knee modules 6 and 7 foot modules form a set of lower modules whose function is to assist the user in his efforts produced with the lower body, in particular when walking or when carrying or moving loads. Note that the hip modules 5 are symmetrical to one another. Hip modules 5 therefore comprise identical or similar parts. Similarly, knee modules 6 are symmetrical with each other, and include identical or similar parts. It is the same foot 7 modules, shoulder modules 3 and 4 elbow modules. hip module As illustrated in Figures 1 to 3, each hip module 5 comprises a femoral portion 51 suitable for being fixed on the thigh of the user and a hip joint 52. The femoral part 51 comprises a femoral segment 1 51 provided to extend along the thigh of the user and fastening straps 512 adapted to surround the user's thigh to fix the femoral segment 51 1 to the thigh. Each hip module 5 is connected to base module 1 through a respective hip joint 52. Specifically, the hip joint 52 connects the portion 51 of the femoral hip module 5 to the belt 1 1 of the basic module 1. Hip joint As illustrated in Figures 8A and 8B, the hip joint 52 comprises a hip actuator 521 to assist the user during a flexion movement or extension of the hip of the user. The actuator 521 includes a stator 522 and a rotor 523 adapted to be rotated relative to the stator 522 when the stator 522 is supplied with electric power for rotating the hip module 5 relative to the base module 1 during a flexion or extension of the hip. The hip joint 52 further comprises an elastic member 524 arranged to exert a return spring force that supports the rotor 523 when the user gets up from a sitting or crouched position. The elastic return element 524 may comprise a pretensioned spring arranged between the stator 522 and the rotor 523, in a guide groove 525 provided between the stator 522 and the rotor 523. More specifically, the elastic return element 524 is arranged so that: - in a first angular range of movement CH rotor 523 relative to stator 522, corresponding to an angular range in which is located the rotor 523 when the user walks or runs (Figure 8A), the elastic member return 524 n 'exerts no spring force on the rotor 523, and - in a second angular range