Added the 'NS 2-site expanded' model to the relax_disp.select_model user function frontend.
This is the numerical model for the 2-site Bloch-McConnell equations expanded using Maple by Nikolai Skrynnikov.
This commit follows step 2 of the relaxation dispersion model addition tutorial (http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
Authored by: bugman 2013-07-17
Parent: [r20359]
Child: [r20361]
