Added the 'NS MMQ 3-site' models to the dispersion auto-analysis.

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#The_auto-analysis.
Fix for the Relax_disp.test_ns_mmq_3site system test.

The wrong spin was being checked.
Updated the docstring of the Relax_disp.test_ns_mmq_3site system test.
Added the 'NS MMQ 3-site' models to the GUI model list.

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#The_GUI.