Add support for "spherical" joint (from PyBullet)

[ManifoldFR]

Addresses issue #8.

Essentially, "spherical" is an alias for the "floating" joint type, with no translation component in the transformation matrix (I added a check for this in Joint.get_child_poses()). This is supported by the Bullet/PyBullet physics simulator and represents joints that have free 3D rotation but fixed origin.

I took care to add comments in the right places I think.

[ManifoldFR]

The new joint type isn't covered by the new unit tests, though. I think the humanoid.urdf robot from PyBullet would be make for a suitable one, no?