relax SVN archive

The centre of mass function now works if the sequence does not exist in the current data pipe.

The spin_num and spin_name have been renamed to atom_num and atom_name respectively in the function
as well.

Modified centre_of_mass() to call a new generator function atom_loop() in the structural data object.

Added a missing import.

Fixed a test for the presence of structural data in the current data pipe.

Fixed the call to the centre_of_mass() function in the N-state model code.

Removed a broken and now useless test.

An initial print out for the parser type.

Removal of a few comments.

The load_structures() method is now properly called.

The cdp.structure object is now set to the Scientific Python structural data object.

Some code has also been shifted into the load_structures() method.