see #43

closes #43

* make false_easting and false_northing optional

* add tests for false_easting and false_northing

* use the convert_coordinate method to transform plots

* update CHANGELOG

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

Fixed
-----

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

Added
-----

-   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](https://github.com/psyplot/psy-maps/pull/35)

* do not set clip path for cartopy>=0.18

* update changelog with fix for #37

* replace RTD everywhere

* use psyplot-ci-orb@1.5.24

* minor fix for test instructions

* fix copyright

* fix email

follow-up of #36

and add disclaimers everywhere

* change config to use circleci and remove reference_figures

* split tests into multiple files

* compatibility fix for gridliners with cartopy>0.18

* install pytest-qt

* revise docs environment

* install psyplot-gui when building the docs

* use python=3.8 for the docs

* fix broken links in docs

* fix badges

* add CI changes to changelog

Remove deprecated matplotlib method from library

According to [matplotlib](https://matplotlib.org/3.2.1/api/api_changes.html) the method 
`validate_path_exists` is deprecated and should be replaced by `os.path.exists`

fix visualization of unstructured data on non-PlateCarree transform

because otherwise we might end up with a transforming projected data

follow-up of https://github.com/psyplot/psy-maps/pull/31

fix plotting of curvilinear data with bounds