feat:add QCD workflow

[hsinweihsia]

1. QCD workflow is pushing back to the git repo which is called QCD_validation.py while it needs to be debugged since the recent correction.py causes issues in QCD_validation.py.
2. selection cuts regarding QCD workflow are added in selection.py.
3. QCD Pt binned samples' Xsec are added in xsection.py.
4. add variables in definitions.py, most of them are related to secondary vertex variables.
5. QCD variables are added in histogrammer.py and plotdataMC.py.
6. Can't use standard PU reweighting files from jsonpog-integration because the HLT path used was highly prescaled. customized PU reweighting root files are prepared for QCD workflow. puweight_2018_UL.HLTPFJet140.histo.root is used for HLT_PFJet140 path (standard). puweight_2018_UL.HLTPFJet40.histo.root was used for low pt study with HLT_PFJet40.
7. prescale files regarding HLT-PFJet140 are added. THe original containing the prescales information is Prescales_PFJet140.csv, and the file directly used for implementing the prescales is ps_weight_JSON_PFJet140.json.
8. The script used for generating ps_weight_JSON_PFJet140.json is added, which is prescales.ipynb. The script was originally provided by Lauren Hay.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[Ming-Yan]

Hi @hsinweihsia
https://github.com/Ming-Yan/BTVNanoCommissioning/tree/QCD
this is the updated PR with merging the changes you made.

I also checked the scale out also work and indeed PU weights are not needed in this case.
If this is good with you. I can open a PR to integrate the changes

[Ming-Yan]

Thanks @hsinweihsia for the contribution. The integration is done in #91 further changes can be made with new PR.