Plot vessel presence with the Spilhaus projection.
See notebooks/PlotPresence.ipynb for an example of downloading some data and plotting it. This is only one year of data instead of several for the plot above and is plotting at lower resolution, so it won't be quite as pretty.
N.B. spilhaus.py and contains GPL-3 licensed code from https://github.com/rtlemos/spilhaus
so this entire repo is GPL-3 licensed rather than our more typical Apache license.