relax SVN archive

All the atom_loop() args in the API have been set to False.

This will make the method more robust as the caller will get exactly what it asks for, i.e. the
tuple will be of the right size.

Made the call to the atom_loop() method in the centre_of_mass() function more robust.

By setting all the flags, the returned tuple should always be the correct size, even if additional
atomic data is specified in the API method.

Updated the call to the atom_loop() method in the centre_of_mass() function.

Made the structural API method atom_loop() much more flexible.

Each part of the tuple of atomic information can be individually selected to be yielded, and the
element name is now also returned.

Bug fix for the calling of terminate().

This is now an object method which is part of the structural data API, rather than a module
function.

Added the missing import of Internal_PDB.

Replaced all references to 'atomic_data' with 'structure'.

Removed a reference to atomic_data in a docstring.

Replaced the initialisation of the atomic_data dictionary with an instance of Internal_PDB.

Added a package docstring.