Added support for the 'MQ CR72' model to the relax_disp.select_model user function back end.

This is the Carver and Richards (1972) 2-site model expanded for MQ CPMG data by Korzhnev et al.,
2004.

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.
Created the 'MQ NS CPMG 2-site' model target function.

This is the Carver and Richards (1972) 2-site model expanded for MQ CPMG data by Korzhnev et al.,
2004.

This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#The_target_function