PERF: Groupby.idxmax
cc @phofl this is currently failing locally and could use a fresh pair of eyes.
I'll try to take a look over the weekend @phofl im mothballing this to clear the queue, just a ping so this doesn't fall off your radar entirely I think the main problem is that ``_wrap_aggregated_output`` isn't working for unordered categories that are empty. All other reductions that are tested are raising TypeErrors, while idxmax and idxmin return the wrong result. Any ideas what we could do there?