relax SVN archive

Corrected calls to the heteronucleus in the model-free system test code.

Updated the __all__ list.

Removed many unused imports.

Created the internal relax PDB object class.

Spun out all of the functions accessing 'atomic_data'.

These will be converted to methods of a new type of internal PDB object.

Corrected errors causing failures of the unit test for consistency testing variables set up.

The variable 'heteronuc_type' was used instead of 'heteronucleus'.

Partially fixed more code so the consistency tests system test can go further before failure...

Set missing parameter values in system test code.

Added the xh_vector() prototype method stub to the structure API.

Partially fixed more code so the consistency tests system test can go further before failure...

Added a missing variable.

Corrected a docstring and a comment.