prevent self-collisions in robot motion planning

[tomsilver]

haven't seen it be an issue yet in practice, but there's nothing preventing self-collisions in the motion planner right now. ideally, the sampler inside of run_motion_planning should sample only joints that are not in self collision, or the collision checker should check for self collisions.

[williamshen-nz]

I think I have it working in my other project. One easy way to check, is that the zero configuration for the Panda is in collision.

Might need to use a specific flag when loading the URDF (use self collisions or something)