Added the 'NS MMQ 3-site' models to the dispersion variables.
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_the_model_to_the_list
