Make all plotting dependencies optional

[ocefpaf]

We should make all plotting dependencies, like pandas_flavor, matplotlib, and cartopy (any other?), optional as a plotting extras.

@FloraSauerbronn this would require you to edit the requirements.txt file, to remove any plotting dependencies that may be listed there, and add a new entry in the pyproject.toml like:

[project.optional-dependencies]
plotting = [list_the_dependencies_here ]

Can you tackle that?