[FEATURE] Reorder categorical axes

[andrzejnovak]

I've hit this while implementing scikit-hep/mplhep#303 and realizing the results can be funny if the categorical axis is set with growth=True. I think it makes more sense to order on the object, than at plotting time. API could look like

h.sort('axis_name', key=list | fcn as passed to sorted())

[andrzejnovak]

Sorry if this should have been opened in bh, still figuring out what goes where.