relax SVN archive

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.

Converted the read_pdb() function to the new relax design.

Fixed the structure user functions for the module rename of r5350.

Updated the __all__ package list.

Renamed the generic_fns.structure.structure module to generic_fns.structure.main.

Docstring improvements.

Spun out the centre_of_mass() function to its own module.

This new module will contain all functions related to mass and inertia.

Fixed the internal call to the atom_add() method.

Made one of the methods private by renaming to __get_chemical_name().

Changed all the docstrings to say 'method' rather than 'function'.