relax SVN archive

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These have been replaced by calls to the is_empty() data object methods.

Deleted the exists_mol_data(), exists_res_data(), and exists_spin_data() functions.

These are now redundant because of the is_empty() methods of the mol-res-spin data object stack.

Reverted r5491, r5492, and r5493 as these introduced a bug.

The command used was:
svn merge -r5493:5490 .

The problem is that empty MoleculeContainers, ResidueContainers, and SpinContainers should be
returned when the selection identifier only selects a higher structure.