add histograms into SignalRegion

Histograms of "rawE" and "wgtE" before & after PU subtraction are filled and stored in output .root files.
The SignalRegion class can be used by:
  SignalRegion SignalEnergy(outFolder, nLayers, nEvts, geomConv, puDensity);
  SignalEnergy.initialise(lSimTree, lRecTree, mycalib, outputFile);
  SignalEnergy.fillHistograms();