implement CI matrix

See merge request !37

implement decoder command-line argument

See merge request !36

fix updating current projects in console

See merge request !35

migrate to psyplot-plugin-template

See merge request !34

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

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

Changed
-------
-   psyplot is now officially licensed under LGPL-3.0-only, see [#29](https://github.com/psyplot/psyplot/pull/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](https://github.com/psyplot/psyplot-gui/pull/28)

* replace links to RTD

* fix license and copyright

* change license to LGPL

* include license change in changelog

* remove misleading requirements file

* add circleci project setup

* fix docs

* update appveyor config

* remove RTD config

* add environment file for ci

* remove redundant files

* update link to documentation

* install psutil for tests

* update changelog with circleci changes

use versioneer for version control

Disable intersphinx on windows by default

Revert jupyter_client restriction

i.e. c8e50da3