calc_diff added: calculates the difference between the reference setup and fiber spool setup PPS measurements

repaired data_vs_fit_err_clean in case of outliers, repaired single file analysis, added extra note in results.txt

Using reference measurement (same setup witouht long fiber) there is no need for pre-initialized hardware delays

Analyze_sellmeier now analyzes each file individually and in a second pass calculates the average of all measurements and determines the tangent from the average