Compatibility fixes and code formatting

Fix density and lonlatbox in mapvector (#45)

Merge pull request #45 from psyplot/develop

Compatibility fixes for psy-simple v1.4.1 (#42)

Fix projection issues

Fixed
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- `false_easting` and `false_northing` are now expected to be optional
  arguments for most projections (see #41)

Changed
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- we now use the `convert_coordinate` method that has been introduced
  in psyplot/psyplot#39 and psyplot/psy-simple#30. See #41

Compatibility fixes and LGPL license

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

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

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

Changed
-------

-   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](#35)

Fix visualization of regular data with plot="poly"

Patch for 3D bounds

New GUI widgets and better projection support

Improved visualization of unstructured grids

New version with improved projection handling

First official and stable release