Hemisphere-centric Direction of Wind Flags on Observations

[bryanguarente]

When looking around the web and when going back to basic met classes, we were taught that the flags on the wind barbs should be pointed to the left in the Northern Hemisphere and to the right in the Southern Hemisphere when facing in the direction the wind is flowing (wind at your back).

This isn't a feature that is easy to control in MetPy unless you set "flip_barb=True" flag on the draw_barb function. This should be latitude-dependent rather than universal for a plot. Is there a way to have this set based on the latitude of a point? So Southern Hemisphere barbs would be appropriately plotted even when showing a Global image?

I understand there are other considerations here like with soundings, the wind barbs on there should be flipped when in the Southern Hemisphere, but you may not have the lat-long points at the time you are making that plot. In those instances, it might not make sense to use the latitude-dependent method.

[dopplershift]

This may make sense to try to get upstream in Cartopy, but I haven't fully decided if this would be easy or hard to do in a general fashion. i.e. given projected coordinates, how hard would it be to tell if we're above/below the equator.

Another problem are data that span the equator--right now that flag in matplotlib is an all-or-nothing affair. Might actually need to do some work upstream to make it more flexible (i.e. flip_barb accept an array of true/false).

@pelson Any thoughts on Cartopy handling latitude-dependent flipping of barbs? (modulo any matplotlib changes that are needed)