ENH: sixDoFRigidBodyMotion. Variable renaming and addition of

initialOrientation.

Rename of IO to more understandable terms.  Rename ref to initial
where correct - ref is to do with the object being in Q = I state,
inital is to do with where things are when the motion starts and
pointDisplacement and value are zero.

Adding initialOrientation so that non Q = I initial states are
correct.