Adds a third option for collision pair removal in to_pin_robot

Previously, there were two options when generating a robot model from an assembly: Either all bodies which are not rigidly connected could collide or collision between those which are directly connected by a joint (and nothing else in between) also could be excluded from collision checks.

This MR adds a third, even more relaxed option: Any two bodies for which there exists a "path" in the robot kinematic that contains at most 1 joint can be excluded from collision checks. This option can be used when one is absolutely sure that the joint limits provided prevent collisions between neighboring modules s.t. collision geometries can be even more over-approximated.