Streamline re-ordering/merging/dropping columns (Rob)

[GluonicPenguin]

So streamline the functions to let user merge columns together, then re-order or drop columns as necessary, for input to fast-plotter, ensuring the index is set for the new merged column (see working code below)

stages:
    - {rename_region: ReBin}
    - {rebin_met: ReBin}
    - {combine_region_met: CombineColumns}
    - {drop_region_met: AssignDim}
    - {save: WriteOut}

rename_region:
    axis: region
    drop_others: true
    mapping:
	0: SR
	6: SB0
	7: SB1
	8: SB2
	9: SB3
	10: SB4

rebin_met:
    axis: met
    drop_others: true
    mapping:
	"[200.0, 300.0)":   "[200.0, 300.0)"
        "[300.0, 400.0)":   "[300.0, 400.0)"
        "[400.0, 500.0)":   "[400.0, 600.0)"
        "[500.0, 600.0)":   "[400.0, 600.0)"
        "[600.0, 700.0)":   "[600.0, 2000.0)"
        "[700.0, 800.0)":   "[600.0, 2000.0)"
        "[800.0, 900.0)":   "[600.0, 2000.0)"
        "[900.0, 1000.0)":   "[600.0, 2000.0)"
        "[1000.0, inf)":   "[600.0, 2000.0)"

combine_region_met:
    format_strings: {"region_met":"{region}_{met}"}
    as_index: [region_met]

drop_region_met:
    drop_cols: [region, category, met]

save: #{}
    filename: "tbl_dataset.region.category.met--sig_regions_fit.csv"