add pileup reweighting weight, sum of weights and filter for non-dalitz events, update file reading (bbyy)

• save pileup reweighting to output.
• save initial sum of weights for all H\to\gamma\gamma events
  • in the di-Higgs and single Higgs MC, both H\to\gamma\gamma and H\to\gamma\gamma^{*} (Dalitz) are generated. The way to correctly normalise MC events is described in HGam doc. Currently in easyjet the sum of weight of all events are used for normalisation.
  • one algorithm to calculate sum of weights (still using mcEventWeights[0]) and one filter to reject dalitz events
• use pathlib(python) to read/find config files.
• update postprocessing script