relax SVN archive

Slight changes to the pivot-CoM PDB representation for the frame order test data.

The internal structural object no longer requires the 'ATOM' PDB record for creating PDB files.

This affects the structure.write_pdb user function.

Created the structure.add_atom user function.

This simply exposes some of the functionality of the structural API to the user. The get_molecule()
API method has also been added to return molecule containers.

Created the Frame_order.test_cam_free_rotor2 system test.

This tests the free rotor frame order CaM test model where the axis is tilted from the pivot-CoM
axis.

Created a second test model for the free rotor frame order model.

In this case, the C-domain does not rotate about the pivot-CoM axis, but an axis tilted 30 degrees
from the pivot-CoM axis.

Fix for the structure.displacement user function.

The kabsch() function now returns the pivot of the rotation as well.

Added the translate method to the structural API.

Synchronised the atom_loop() method args in the structural API.

Updated the generic_fns.structure package __all__ list to include the superimposition module.

Fix for the structure.translate user function front end.

The back end already exists, so this is now functional.