relax SVN archive

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.

Reverted r5501, as the behaviour of the tested code has been reverted to the original.

The command used was:
svn merge -r5501:5500 .

Fixes for the deletion of the exists_xxx() functions.

These have been replaced by calls to the is_empty() data object methods.