Added support for the 'NS MMQ 3-site' model parameters to the lib.text.gui module.
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
