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.
Added support for the new 'dwH' dispersion parameter.

This is needed for the 'MQ NS CPMG 2-site' model support.

This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#Adding_support_for_the_parameters.
Improved the RelaxError from the relax_disp.exp_type user function when an invalid experiment type is set.