relax SVN archive

Updates for the new cone PDB file where the negative image is in a second model.

Fix for the test___parse_pdb_record unit test.

The private methods have been renamed.

Fixes for the add_molecule() structural API method.

Fix for the internal structural object add_molecule() method for when no models exist.

Now a model is created if none currently exist.

More use of the structural API in the create_diff_tensor_pdb() function.

A lot of fixes for the internal structural object for handling multiple models.

The positive and negative cones from the frame order theory are now different PDB models.

Clean up of a RelaxError print out.

Fixes for all the renamed private methods in the internal structural object.

Created the add_model() structural API method.

This is implemented within the internal structural object.