HistogramsUser base and inspection tasks

[mafrahm]

This PR implements base tasks for loading histograms in Columnflow + two tasks that implement these base tasks for debugging purposes and as a simple exemplary usecase.

This task call loads histograms for only the nominal shift

law run cf.InspectHistograms --version v1 --variables jet1_pt --shift nominal --processes tt_sl

This task call loads histograms for nominal, murf_envelope_up, and murf_envelope_down

law run cf.InspectShiftedHistograms --version v1 --variables jet1_pt --shift-sources murf_envelope --processes tt_sl

[pkausw]

Looks good, thanks @mafrahm ! I think this will make e.g. evaluating ABCD methods much more feasible. It would be great if you could implement the small changes that I mention inline 😬

[pkausw]

Just an even more minor comment, almost done 👍

[pkausw]

Thanks 👍