Extended Multidof support in Pybullet

[ManifoldFR]

Following my issue #2907, I would like to extend support for Multidof (spherical) joints.

I think these would be the main points:

• add support for calling the pb.calculateJacobian() function on characters with spherical joints,
• possibly add support for IK (DLS) for such characters

I will look into it and make a PR if I get things to work.

[ManifoldFR]

As a first step, I will expose the b3GetNumDofs function from C-API as pb.getNumDofs, which will return the number of Dofs as computed in the btMultibody class. This would be separate from pb.computeDofCount() which computes on the fly using b3ComputeDofCount but does not support spherical/planar joints.