Fix a bug for double-counted events and first setup for combine

I fixed a bug to make sure that the DATA events only will be treated to remove the double-counted events. I also started to setup everything for combine: I added new histograms with the correct name and weight to create a dedicated root file with combine_rootfile.py. This root file will be the input file to create the datacard with card_creator.cpp (taken from Nicolas' code, it has to be executed with root via 'root -l card_creator.cpp' without any compilation).