Renamed all of the current numeric dispersion models in relax to be specific to CPMG-type data.

This is in preparation for adding R1rho numeric models.  It was proposed at
http://thread.gmane.org/gmane.science.nmr.relax.devel/4461.
Added the 'NS R1rho 2-site' model to the specific_analyses.relax_disp.variables module.

This is the numerical model for the 2-site Bloch-McConnell equations for R1rho data.

This commit follows step 1 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).