relax SVN archive

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'.

Added a module docstring to the relax internal PDB structural object.

Included generic_fns/structure/ in the API docs.

Improvements to the generic_fns.structure package docstring.

Added the write_pdb_file prototype method stub to the structral data API.

All the PDB record documentation has been shifted to the API stub method.

A number of docstring updates for the structural data API.