Create JointPrims for fixed joints too, distinguish fixed and controllable joints via a controllable_joints member on controllable_object

[cgokmen]

Currently we can't create JointPrim objects for fixed joints because they show up in the .joints member of robots etc. which complicates a lot of math due to their 0 degrees of freedom.

Let's avoid this filtering by either adding a new member that only shows controllable joints and using it, OR even better, making all joint-based code work with variable #dof instead of just 1 dof per joint.