relax SVN archive

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.

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

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

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

Removed some useless epydoc text.

Updated the internal PDB structural object to use the self.structural_data object.

The atomic_data structure has been replaced by self.structural_data, and is no longer passed into
each method. The description of this object has also shifted from the methods into the class
docstring.

Corrected some comments.

Corrected calls to the heteronucleus in the opendx system test script code.

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