Refactoring PostProcessing code

I noticed that bbyyPostProcess.py, bbllPostProcess.py, and ttHHPostProcess.py had the same script merging function, so I made it a common function in a new EasyjetPlus/python folder. I also added the option to merge to the output file as personally I wanted to keep my input file untouched, and Giuseppe noticed that lots of the histograms (non-AnalysisMiniTree objects) from the input file get removed if you merge to input (maybe that is fixable via the Snapshot method options.