Added the 'CR72 red' model to the specific_analyses.relax_disp.variables module.
This is the Carver and Richards 1972 analytic model with the simplification R20A = R20B. The
current 'CR72' makes the same assumption, but that model will be expanded to support R20A and R20B
later.
This commit follows step 1 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
