mshumko/aurora-asi-lib: Added plot_map() to project images onto a geographic map

The main addition to this release is a new function asilib.plot_map(). This function maps each pixels in the fisheye image to a map of North America. This function is only tested with the THEMIS ASIs, but should work with REGO since the data format is identical. I will add more unit tests for both REGO and THEMIS in the next patch release.

Now cartopy is a mandatory dependency of aurora-asi-lib. I added the cartopy install instructions for Linux and Mac in the online documentation (I don't work with Windows much, but aurora-asi-lib runs on Window's WSL). Lastly, I added the plot_map() API reference and an example in the online docs.