Automate the selection of torque idx when computing torques

This line is problematic in SimpleGear class:

        act_data.tau[act_model.idx_vs[self.idx_js[0]]] = actuator_data.torque

Because we have to select the torque idx to be updated. There is three cases to manage:

• One actuator for three torques 1 -> 3
• Three actuators for three torques 3 -> 3
• Five actuators for thre torques 5 -> 3 (Overactuated - muscle case)