Add projections to MapPlot

[bathoorn]

Add following projections to MapPlot

• ORTHOGRAPHIC
• STEREOGRAPHIC (similar to STEREO_NORTH and STEREO_SOUTH but you set the center of the projection)
• ZENITH (Stereographic centered on your own location show visible sky)

added examples for each

• in zenith plot the dotted red line is the horizon

[steveberardi]

@bathoorn I really like what you did here! I've been wanting to refactor Starplot's ZenithPlot to use Cartopy in the background (like MapPlots), to make it easier to plot things in a similar way across plot types, and the code here is a big step in that direction. Thanks so much for contributing this! :)

I did notice plotting the Zenith plots in this way does significantly increase the plotting time (about 8x slower), but I think there's some simple things we could do to improve that (e.g. limit the star query to only include those that are visible, etc). If you have any other ideas, or wanna help out with that too, that'd be great!

Anyway, I'm gonna merge this PR as-is since it's just going into another development branch. I'll probably re-work some of the code after that, and hopefully have some of this polished up for the v0.9.0 release.