[Python] Fixes in MbsSensor class:

  • bad field called in comp_f_sensor raising exception.
  • bad field called in assign_to_C raising exception when assigning an existing Python sensor to an existing C structure.
  • with C symbolic libraries computing force or gen sensor was wrong as the class calls for the Python function.
  • gen_sensor function is renamed to comp_gen_sensor (coherence with other sensors).