Various extension to the roo fitter

[saganatt]

• save chi2 values in the output files
• save fit functions in the output files
• add protections against crashing
• add the possibility to set fixed sigma values when fitting MC data

NOTE: It extends the fit_mass_new() arguments and outputs, so all analyzer classes would need to be updated.

FYI @DelloStritto

[saganatt]

The remaining MegaLinter errors come from issues like duplicate code and string formatting. Unrelated to my PR.