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.
The code in common with the CR72 model has been shifted into the new calc_CR72_chi2() method.
This commit follows step 4 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
