relax SVN archive

The calculation of PCS unit vectors and distances has been shifted into the new
maths_fns.paramag_centre module and into the N-state model maths_fns code. The API of
maths_fns.n_state_model has been changed so that now atomic positions, the paramagnetic centre
position, the temperature, and frequency are passed in rather than the PCS vectors and constants.
The code is now more flexible making it easy to add PRE support.