Fix bug in python bindings of InverseDynamicsFormulationAccForce. The methods getContactForces and checkContact were assuming that the contact force vectors were always 12d. This was the case a while ago, but now TSID supports also 3d contact points, so this assumption does no longer hold in general. To fix the bug I have introduced a new method that allows the user to get the contact force vector without knowing its size in advance.