Created the Relax_disp.test_ns_mmq_3site_branched system test.
This is for the 'NS MMQ 3-site (branched)' CPMG dispersion model.
This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#The_test_suite