relax SVN archive

Expanded the Structure.test_displacement() system test to test the XML save files.

The structural model displacement objects are now stored in the relax XML save files.

Implemented the back end of the structure.displacement user function.

This will still require some debugging, and the data objects will need to be stored in the XML save
files.

Created the Structure.test_displacement() system test for checking the structure.displacement uf.

Added the atom_id arg to the new structure.displacement user function front end.

This will be used to restrict the calculation to subsets of all atoms.

Created the front end for the new structure.displacement user function.

This is used to calculate the rotational and translational displacement between two models.

Extended the N-state model analysis to allow a separate paramagnetic centre per state.

Bug fix for the pymol.cone_pdb user function for when there is no average domain position.

This is necessary for the old 2-domain N-state model analysis type.

Bug fixes for the n_state_model.cone_pdb user function.

The code was old and not functional any more.

Added checks for the n_state_model.cone_pdb user fn in the N_state_model.test_5_state_xz system test.