Draft: Add basic planar joint support in the urdf parser

[mpowelson]

Levi and I discussed how to implement this. The better solution would probably be to implement planar joints in the state solver. However, that would touch a lot of things. Instead we decided to get an MVP working by inserting 3 joints for each planar joint in the URDF. They are named <urdf_name>_<x, y, or yaw>_planar. Then the ROS environment monitor will listen to TF and update these TFs as appropriate

[Levi-Armstrong]

I been looking at how MoveIt accomplishes this and think it aligns with what we would like to do. In this case add the concepts of variables where a joint can have multiple variables. A good point of reference is the joint model here.