relax SVN archive

Preparation for the optimisation of the paramagnetic centre in the N-state model.

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.

Bug fix for the RDC back_calc() function - the align_id arg was being ignored.

The RDCs were being calculated for all tensors, and then assigned to the align_id key. The results
was that the back-calculated RDCs were for the last tensor, not for align_id.

Fix for the RDC back_calc() function for when N=1 in the N-state model.

The bond vector structure was not correct.

Fixes for solely specifying that the paramag centre will be optimised.

Activated the paramag centre finding system test.

This currently fails as the code to do this does not exist.

Fixes for the system tests for the pcs.centre() to paramag.centre() shift.

The paramag user function class is now accessible.

A few fixes for the paramag prompt code for the change from the PCS.

The fix flag is now accepted by the paramag.centre() user function.

Renamed the pcs.centre() user function to paramag.centre() to abstract for the PRE.

The paramagnetic centre is shared between different NMR interactions, so this function has been
abstracted and is no longer PCS dependent.