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.
Redesigned the Relax_disp.test_hansen_cpmg_data_to_ns_cpmg_2site_star_full system test.

The R2A and R2B rates cannot be distinguished for this data, therefore there was no unique solution.
This resulted in too much variability between 32 and 64-bit systems as well as different operating
systems.  Instead a single calc user function call is used to determine the chi-squared value for a
fixed set of parameters.
Loosened the test_hansen_cpmg_data_to_ns_cpmg_2site_star_full system test for Mac OS X.

Even the calc user function does not help, the results are quite different between different systems.