Pile-up corrections for MC

[katilp]

(from #7 - @slaurila )

Pileup corrections for MC: Three steps are needed to apply pileup corrections to MC in the HTT analysis:

• Publishing the pileup distribution jsons for 2011/2012 data in the open data portal (could be skimmed to contain only the runs published as open data?). The pileup jsons can be found in here.
• Adding the number of interactions (TrueNumInteractions variable, see here) to open data NanoAOD
• Implementing the reweighting in the HTT analysis code: first calculate the PU distribution in data by combining the pileup json with the golden json, then extract the PU distribution from MC samples and calculate the weights as a function of TrueNumInteractions, finally apply these weights to MC