[JOSS] Missing declaration of optional dependency on cartopy

[platipodium]

Neither conda nor pip install cartopy as a dependency. This leads to garbled plots on my machine, but you do provide a helpful message

Unable to import cartopy. For better plots install cartopy or check cartopy installation!

Maybe this should be documented.

openjournals/joss-reviews#5494

[robertjwilson]

Was the garbled plotting not due to an issue with a major update to the panel package and not cartopy?

The installation docs explained the cartopy issue, but very well. I've just added a "How to get better plots" section to installation. https://nctoolkit.readthedocs.io/en/latest/installing.html

That should clarify things for users do a new install. Ideally cartopy would be an explicit dependency on conda, but having it slows the installation down enormously. If everyone used mamba I could switch it.

I've closed the issue, as this is hopefully now fully clear in the docs.