Added linear constraints for the pivot and average domain translation frame order parameters.
The pivot coordinates are constrained between -999 and 999 Angstrom and the translation between -500
and 500 Angstrom. This allows the frame_order.pdb_model user function to operate in the case of
failed models - often the free rotors fitting to torsionally restricted data - by preventing the PDB
coordinates from being out of the PDB format range. It should also speed up optimisation by
stopping the optimisation of failed models earlier.
