Expose method for setting multibody joint position

[jsatka]

Exposing the set_free_pos method publicly is necessary in order to initialize a multibody joint in different position than the reference zero-position of the joint represented by the local frames.

This is necessary e.g. in robotics simulation, when a robot needs to be initialized in various postures represented by joint rotation deltas with regard to a default reference posture, which represents zero-positions of joints.