relax SVN archive

A number of fixes for alignment tensors in the N-state model.

Fix for the n_state_model.select_model user function for the set up of alignment tensors.

The align_id should be set rather than tensor name!

Fix for the N-state model optimisation.

The target function Dij_theta variable is now called rdc_theta.

Fix for the get_pos() function for the change in the atom_loop() API method.

Fixes for the structure.superimpose and structure.displacement user functions.

This is needed for the new atom_loop() API design.

Redesigned the model looping in the atom_loop() and base_vectors() API methods to fix a bug.

The models are now looped over in numerical order, rather than the order they were loaded or
created. This reverts to the original behaviour of the main line.

Updated the test_atom_loop_spin_selection2() unit test for the redefined atom_loop() method.

The centre_of_mass() function now averages the atomic positions over all models.

This is to fix a bug with the change to the atom_loop() API call.

Fix for the Align_tensor.test_to_and_from_xml() system test.

The domains are now defined.

The domain user function arg spin_id can now be set to None.

This is useful if the domains are not associated with spins.