Compute dotS matrix for free-flyer joint as it is not zero.

  * The free-flyer joint S matrix depends of the configuration vector,
    which means it apparent time derivative dotS is not zero.
  * The time derivative of a rotation matrix R is skew(w).R, where w
    is the angular velocity. We retrieve w from the velocity
    configuration vector qi.
  * The joint acceleration term cj is then computed as being dotS.dqi
Update simple_humanoid rnea reference file.

  * This is done in compliance with the dotS and cj terms computation
    in the free-flyer joint.