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The labels are created by the frame_order.pdb_model user function backend. For the double rotor
model, these are 'x-ax' and 'y-ax'. For all other models, the label is 'z-ax'.

The labels are then sent into the lib.structure.represent.rotor.rotor() function via the new label
argument. This function adds two new atoms to the rotor molecule which are 2 Angstrom outside of
the rotor span and lying on the rotor axis. These then have their atom name set to the label. The
residue name is set to the new RTL name which has been added to the internal structural object
_get_chemical_name() method to describe the residue in the PDB file for the user.

Finally the pymol.frame_order user function selects these atoms, hides them and then labels them
using the atom name (x-ax, y-ax, or z-ax).