Add single top modeling systematics

[alexander-held]

Use the existing tW variations to define modeling systematics. cc @ekauffma

Relevant input file excerpt:

    "single_top_tW": {
        "nominal": {
            "nevts_total": 1999400,
            "files": [
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_antitop_5f_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19412_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_00000_0000.root",
                    "nevts": 847600
                },
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_antitop_5f_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19412_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_20000_0000.root",
                    "nevts": 151800
                },
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_top_5f_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19419_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_70000_0000.root",
                    "nevts": 1000000
                }
            ]
        },
        "scaledown": {
            "nevts_total": 995200,
            "files": [
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_antitop_5f_scaledown_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19415_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_20000_0000.root",
                    "nevts": 495200
                },
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_top_5f_scaledown_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19422_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_10000_0000.root",
                    "nevts": 213600
                },
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_top_5f_scaledown_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19422_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_80000_0000.root",
                    "nevts": 286400
                }
            ]
        },
        "scaleup": {
            "nevts_total": 998800,
            "files": [
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_antitop_5f_scaleup_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19416_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_20000_0000.root",
                    "nevts": 499600
                },
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_top_5f_scaleup_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19423_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_60000_0000.root",
                    "nevts": 499200
                }
            ]
        },
        "DS": {
            "nevts_total": 1140600,
            "files": [
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_antitop_5f_DS_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19410_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_00000_0000.root",
                    "nevts": 260600
                },
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_antitop_5f_DS_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19410_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_50000_0000.root",
                    "nevts": 238600
                },
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_antitop_5f_DS_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19412_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_20000_0000.root",
                    "nevts": 151800
                },
                {
                    "path": "https://xrootd-local.unl.edu:1094//store/user/AGC/nanoAOD/ST_tW_top_5f_DS_inclusiveDecays_13TeV-powheg-pythia8_TuneCUETP8M1/cmsopendata2015_single_top_tW_19417_PU25nsData2015v1_76X_mcRun2_asymptotic_v12-v1_10000_0000.root",
                    "nevts": 489600
                }
            ]
        }
    }