Plotting horizontal slices and map projections

[kshedstrom]

I have always been grumpy about the pyroms_toolbox zview code. In particular, zview is not only computing the fields on a particular z surface, but it is also doing the plotting internally. I have a number of problems with this:

• It works for either Mercator projections or "None", not my usual conic or polar stereographic.
• I was doing the actual plotting with some software other than matplotlib.
• It works for some default color scheme and line styles, etc. Controlling that stuff should be outside of a generally callable toolbox routine. Or was it meant to be copied and hacked?
• Bror has some tools for managing your map projection collection (projmap) - maybe we can start using that or something like that, just like we have the gridid.txt file.