relax SVN archive

Shifted some code from InteratomList.add_item() to generic_fns.interatomic.create_interatom().

This is to break a circular import problem.

The rdc.read user function backend is now adding the RDCs to the interatomic data containers.

Created the generic_fns.interatomic.create_interatom() function for creating interatomic data containers.

Added checks to the InteratomList.add_item() method to make sure that the spin IDs already exist.

Fix for the InteratomList.add_item() method.

Created the generic_fns.interatomic module and added the return_container() method.

Created the InteratomContainer.id_match() method for checking the spin IDs in both directions for a match.

Converted a few of the N-state model system tests to use the new rdc.read definition.

Added the script for the system test of the last commit (r16856).

Created an initial system (and GUI) test for the absolute RDC concept.