[IMP-dev] atom::setup_as_rigid_body

There is some bug but with setup_as_rigid_body which makes is not work correctly with Selections (the selection will only choose the root particle under certain circumstances when it should choose the leaves as some of the internal particles are given a radius of 0). Since the method is not documented or supported and has other areas where it doesn't work right, I'm not terribly motivated to fix it and would rather simply remove it. Thoughts?