BTag SF

This is a first implementation of the btag weights and uncertainties. See the Twiki for the logic of this: https://twiki.cern.ch/twiki/bin/viewauth/CMS/BTagSFMethods. We use method 1a, unlike the previous version of the analysis.

The SF are taken from the csv file using the BtagCalibrationStandalone code.

Efficiencies are computed within the framework of our analysis (the histos are produced anyway): you can re-compute them yourself by running the extractBeff.C macro, but the results obtained have also been stored in data/efficiencyTables. Notice that I have used ZZ->2l2nu samples to produce the efficiencies in this MR; this could be changed or optimized in the future.

The efficiency tables use a format shamelessly stolen from the SMP group. It is encouraged to use this format from now on for other SF or efficiencies, as it is much more handy than having hardcoded numbers in a function... The related codes are in Common/Tables.cc.

The uncertainties on SF have been implemented (these come only from SF and not efficiency, following the recommendation of the local expert). See those 2 new lines on the systList.txt file.

The uncertainties are anticorrelated, because btagging is used as a veto. I had to implement this in the yield code (which crashed previously). I found it easier to rewrite a part of the logic of this code, so that cases where this happen don't need to be treated as exceptions.

Nothing has been added yet for the datacards, I'm looking into it. Also, I still have to check some details about the yields; hence, the changes in prepareDataCards.py are preliminary.