Implements functionality of ik as described in docstring

The IK implementations had some bugs:

• IK scipy did not actually perform a proper random restart. Instead, the same q_init was used over and over again
• The allow_random_restart kwarg was ignored in ik_scipy
• The logic for allow_random_restart in ik_jacobian was implemented the wrong way round: If it was set to False, the random restart was done anyways.