Add pileup reweighting to ZFinderEvent

We use the standard CMS method of calculating pileup reweighting for MC
and store it as an event weight. The event weight is then used as the
base weight when applying efficiencies from cuts.
Add pileup weighting scripts

pileupCalc.py is from CMSSW, but it is convenient to have in our
repository as well.

histo_to_vector.py prints out the bins in a histogram, like the ones
produced by pileupCalc.py, so that they can be turned into a vector that
is used in the reweighting code.