relax SVN archive

For this the new 'num_points' and 'extend' arguments have been added to the user function to give
the user better control of this plotting. The interpolated curve is disabled from the numeric CPMG
models as these do not support interpolation, and the 'R2eff' model as interpolation is not needed.

To support this, the specific_analyses.relax_disp.optimisation.back_calc_r2eff() function has been
extended to support the CPMG frequencies or spin-lock field strengths been supplied instead of
retrieved. This allows a set of custom dispersion points to be used in the back calculation.

The dispersion target function setup was modified to prevent the recalculation of tau_CPMG values
when asked, as interpolation is not compatible with this.