relax SVN archive

Shifted the create_diff_elem() fn to the DiffTensorData class.

Renamed a number of variables and methods.

Converted the create_hybrid_elem() fn to the PipeContainer.xml_hybrid_elem() method.

Fixes to the PipeContainer.xml_write() method.

Shifted the contents of create_top_level() into Relax_data_store.xml_write().

Shifted most of the PipeContainer.xml_write() method to the Relax_data_store.xml_write() method.

Fixed some import statements.

Updated the Relax_data_store.__repr__() method.

Shifted and renamed the XML module.

Shifted the xml import statements.