Created the 'NS 2-site star red' model target function.
This is the model of the numerical solution for the 2-site Bloch-McConnell equations using complex
conjugate matrices, whereby the simplification R20A = R20B is assumed. The code in common with the
'NS 2-site star' model has been shifted into the new calc_ns_2site_star_chi2() method.
This commit follows step 4 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
