relax SVN archive

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.

Converted some of the test suite RDC data to use the double spin ID format for interatomic data.

Converted the only sample script which uses the rdc.read user function to the changed definition.

Converted the rdc.read user function backend to the new interatomic data design.