Refactoring of 2pc code

[Marcellocosti]

@wuctlby

[wuctlby]

Hi @Marcellocosti , thanks a lot for the work! I believe you have already tested it locally, so it looks good to me. Just one minor comment,
for correlations/PostProcessing/FitCorrel.py to correlations/PostProcessing/fit_correl.py
and correlations/PostProcessing/construct_v2_mass.py to correlations/PostProcessing/extract_ry_triggers.py.
Thanks for the refactoring. I would suggest renaming them rather than deleting the original file, then creating a new one. It's hard to trace what we did in the past and validate the underlying logic