Modified the dispersion auto-analysis to check if peak intensity errors have been pre-calculated.

This allows the user to perform custom analyses and the auto-analysis will then not overwrite these
values.
Bug fixes for the averaging of peak intensity errors in the dispersion analysis.

This is in the specific_analysis.relax_disp.disp_data.average_intensity() function.
Fix for the docstring formula in lib.dispersion.two_point.calc_two_point_r2eff_err().