Compatibility and bug fixes, cli improvements and code formatting

Compatibility fixes and LGPL license

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

Added
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-   psyplot-gui does now have a CITATION.cff file, see https://citation-file-format.github.io

Changed
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-   psyplot is now officially licensed under LGPL-3.0-only, see [#29](psyplot/psyplot#29)
-   Documentation is now hosted with Github Pages at https://psyplot.github.io/psyplot-gui. Redirects from the old documentation at [https://psyplot-gui.readthedocs.io]{.title-ref} have been configured.
-   We use CicleCI now for a standardized CI/CD pipeline to build and test the code and docs all at one place, see [#28](#28)

Presets and more variable info

Improved OpenGL support

Minor update with improved tests

Python3 release with modifiable central widget

Patch for ipykernel 5.1.1

New release for python 3.7

New plugins and formatoptions interfaces

Fixed startup on Windows