Added the 'MQ NQ CPMG 2-site' model to the dispersion variables.

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

The new dispersion variable MODEL_MQ_NS_CPMG_2SITE has been added.  As this is a new data type,
multi-quantum CPMG, the new MODEL_LIST_MQ_CPMG and MODEL_LIST_MQ_CPMG_FULL lists have been created.