Added the 'MQ NS CPMG 2-site' model to the relax_disp.select_model user function frontend.
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_front_end
Two new sections were added to the user function docstring for the MQ CPMG and MQ R1rho experiment
types.
