Add regional plot climatology capability

[megandevlan]

As described in #368 (comment), enables the ability to look at regional climatologies.

Admittedly there's still more to do, but this is a start! A few notes:

• On the website, I'd like the regions to show multi-row, rather than extending the website so far horizontally; not sure how to do that!
• I've commented out some regions in config_clm_unstructured_plots.yml; they break the script and I haven't tracked down why. But since we want to move to yml file for region definition anyway, maybe we wait for that?

[brianpm]

Hi @megandevlan -- I haven't looked through your code yet. I'm not sure if this will be helpful at this point, but I had previously added similar capability, I think. Take a look at: scripts/plotting/regional_map_multicase.py

[megandevlan]

Thanks Brian! I took a quick look initially and opted to diverge slightly, but it's a good point that there might be something more efficient. I can dig into this a bit more (though I might be a bit slow :( )