Added support for the new 3-site exchange dispersion parameters.
This is for the 'NS MMQ 3-site' and 'NS MMQ 3-site (linear)' CPMG dispersion models.
This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#Adding_support_for_the_parameters.
