Fixes for the calc user function for the dispersion analysis.
This now does something logical for the non-R2eff models. The chi-squared value is now being
calculated and stored. Previously this was only calculating the R2eff/R1rho values for fixed
relaxation time period data for the 'R2eff' model and failing for all others. Now the pre-existing
_back_calc_r2eff() method is used to back-calculate and store the chi-squared value.
