Added the 'NS R1rho 2-site' model to the relax_disp.select_model user function frontend.

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

This commit follows step 2 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).