Support for 'spherical' joint type

[ManifoldFR]

I think the "spherical" joint type is not part of the standard URDF spec.

However, the Bullet simulator and its Python API support this joint type along with others such as revolute, but does not support floating although they should be functionally the same for rotation with fixed joint position.

Here is an example of this joint type being used in the PyBullet examples. I think I have a (naive) patch for this where 'spherical' is just an alias for 'floating'.