Added support for the 'NS R1rho 2-site' model to the relax_disp.select_model user function back end.

This is the numerical model for the 2-site Bloch-McConnell equations for R1rho data.  The code
originates from the funNumrho.m file from the Skrynikov & Tollinger code (the sim_all.tar file
https://gna.org/support/download.php?file_id=18404 attached to https://gna.org/task/?7712#comment5).

This commit follows step 6 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).