Fix for return_index_from_disp_point() for when R2eff/R1rho data is loaded rather than intensities.
This specific_analyses.relax_disp.disp_data.return_index_from_disp_point() function was always
subtracting 1 from the dispersion point index to take the reference spectrum into account. This
however fails if R2eff/R1rho data is loaded instead.
