Visualizing MulEnsembleGeoSeries objects

[CommonClimate]

There is currently no mapping capability for this class, because it is not a child of MultipleGeoSeries. I suggest copying over the mapping function from that class.

A stackplot() is in the works (see #579)

It might also be useful, though not essential, to adapt time_geo_plot(). Incidentally @alexkjames , is this function highlighted anywhere in the tutorials?