MulEnsGeoSeries stackplot

[alexkjames]

Adds stackplot functionality to MulEnsGeoSeries class, alongside unit tests. Addresses #579

[CommonClimate]

Hi @alexkjames, thanks for doing this.
I only tested the docstring example, which is fine, though it doesn't show much about how to customize the plot. Maybe refer to MultipleSeries.stackplot() in the "See Also" section so interested users can see how it's done?

[CommonClimate]

I like the new style feature. Can you make sure users can pass ensemble plot options as they would to EnsembleSeries.plot()? In particular, some users will want to adjust shade_alpha for plot_envelope() and num_traces for plot_traces()

[alexkjames]

I like the new style feature. Can you make sure users can pass ensemble plot options as they would to EnsembleSeries.plot()? In particular, some users will want to adjust shade_alpha for plot_envelope() and num_traces for plot_traces()

These are currently adjustable via the plot_kwargs option. I'm a bit hesitant to include exposed arguments for the plotting methods, since there usage will depend on what style is set to. I've include a docstring example of using the plot_kwargs argument to make these adjustments. Will that suffice? Or would you still like certain arguments to be exposed?

[CommonClimate]

Thanks for the changes! Good to go!