Added support for the 'MQ NS CPMG 2-site' model to the relax_disp.select_model user function back end.
This is the numeric solution for 2-site exchange for multi-quantum CPMG-type data.
This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#The_relax_disp.select_model_user_function_back_end.
