psy-maps: A psyplot plugin for visualization on a map

As with psyplot 1.4.0, psy-maps is now continuously tested and deployed with CircleCI.

• psy-maps is now officially licensed under LGPL-3.0-only, see #36
• A bug has been resolved that crash makes unstructured plots fill the entire area on zooming in, see #40
• psy-maps is now compatible with cartopy 0.19 and 0.20

• psy-maps does now have a CITATION.cff file, see https://citation-file-format.github.io

• Documentation is now hosted with Github Pages at https://psyplot.github.io/psy-maps. Redirects from the old documentation at https://psy-maps.readthedocs.io have been configured.
• Examples have been removed from the psy-maps repository as they now live in a central place at https://github.com/psyplot/examples
• We use CicleCI now for a standardized CI/CD pipeline to build and test the code and docs all at one place, see #35