(WIP) ENH/CLN: add HistPlot class inheriting MPLPlot

Because ``hist`` and ``boxplot`` are separated from normal ``plot``, there are some inconsistencies with these functions. Looks better to include them to ``MPLPlot`` framework.

Maybe ``scatter`` and ``hist`` can be deprecated in 0.15 if ``MPLPlot`` can offer better ``GroupBy`` plot (plan to do in separate PR).
needs to add this to the doc strings for ``plot_series/frame``, no?