[IMP-dev] faster RigidBody::get_reference_frame()

Hi, right now RigidBody::get_reference_frame() is a common yet expensive operation (from profilng), especially within scores. The reason seems to be that IMP saves the quaternion, but not the rotation matrix.

Would it make sense to save a cache of the rotation matrix that would update if needed? Or is it possibly that memory cost would be too high?