Subsamples implementation

Adding the possibility to define subsamples as sample specific cuts. Histograms are created separately for each subsample.

"dataset" : {
        "jsons": ["datasets/signal_ttHTobb_local.json",
                  "datasets/backgrounds_MC_TTbb_local.json",
                  "datasets/backgrounds_MC_ttbar_local.json",
                  "datasets/DATA_SingleMuon_local.json",
                  "datasets/DATA_SingleEle.json"],
        "filter" : {
            "samples": [
              
                "TTbbSemiLeptonic",
            ],
            "samples_exclude" : [],
            "year": ['2018']
        },
        "subsamples": {
            "TTbbSemiLeptonic":
            {
                "tt+3b": [get_nObj_eq(3, coll="BJetGood")],
                "tt+4b": [get_nObj_eq(4, coll="BJetGood")],
                "tt+5b": [get_nObj_eq(5, coll="BJetGood")],
            }
        }
    },