PROTOTYPE for support for rigid body kinematics. I'm not sure if supporting this is something that is feasible or desirable for trimesh, this is just exploring what it might look like. If this improves, it could feasibly live as trimesh.kinematic.
Current state: loads an OpenRAVE XML file and moves a robot around between joint limits using sympy lambdas for forward kinematics.
Challenge: create a data structure which can (mostly) hold and cross-convert OpenRave XML, and URDF. Ideally would be lossless between the same format for inport and export.