relax SVN archive

Shifted out the Kabsch superimposition code into its own module.

Added the centroid arg to the structure.displacement user function for pivoted motions.

By specifying the centroid position for all structures, a pivoted motion is assumed. All
displacements will be zero, and the angles will be that of the pivoting motion.

The Structure.test_displacement() system test is now checking all of the numerical results.

Fixes for the structure.displacement user function.

The rotations are now correctly calculated - the problem was that the numpy SVD is not transposing
the last matrix v. The technique is now labelled as the Kabsch algorithm.

The structural model displacement data structure is now restored from the XML save files.

There are a few fixes for the to_xml() methods as well.

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.