Histogramming with multiple shifts (including JECR)

[JulesVandenbroeck]

This is a pull request that will stay as a draft and is intended as a guide on how to modify columnflow to have the option to create histograms of multiple shifts in a single task

HOW TO USE:

• add collection shifts that encapsulate multiple shifts you would like to bundle in the config:

    def add_shift_collection(config: od.Config, name, idx, shift_names):

        shifts = []
        for idr, direction in enumerate(shift_directions):
            col_name = f"{name}_{direction}"
            if config.has_shift(col_name):
                continue
            subshifts = [
                s for s in config.shifts
                if law.util.multi_match(s.name, shift_names)
                if s.name.endswith(direction)
            ]
            col_shift = config.add_shift(
                col_name,
                id=(idx := idx + 1),
                type="shape",
                tags=["is_collection"],
                aux=dict(subshifts=[s.name for s in subshifts]),
            )

            shifts.append(col_shift)

        return shifts
        
    base_collection_shifts = add_shift_collection(
        config,
        "base_shift",
        idx=10000,
        shift_names=base_shifts,
    )     

• In the case that a shift that is part of the collection shift is selection dependent, that shift must have the tag selection_dependent
• Use the hist-producer called shifts_hist with the relevant collection shift variation (eg.: base_shift_up). This will produce a histogram with all subshifts included.
• To use these histograms in cf.CreateDatacards make sure to set all parameters that include shifts encapsulated by the collection shift to is_dynamic. A dummy parameter needs to be added using the collection shift in order for the task to require the relevant cf.MergeShiftedHistograms tasks.
• These histograms can also be used for the cf.PlotShiftedVariables1D task without any issue. This is however currently not possible for the cf.PlotShiftedVariablesPerShift1D task.

If you have any questions don't hesitate to contact @maadcoen or myself (@JulesVandenbroeck)