add buttons to control the viewport

See merge request !66

Background image options

See merge request !65

migrate to psyplot-plugin-template

See merge request !64

Fixed
-----
-   psy-view is now compatible with psyplot 1.4.0

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

Changed
-------
-   psy-view is now officially licensed under LGPL-3.0-only, see [#58](https://github.com/psyplot/psy-view/pull/58)
-   Documentation is now hosted with Github Pages at <https://psyplot.github.io/psy-view>. Redirects from the old documentation at [https://psy-view.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 [#57](https://github.com/psyplot/psy-view/pull/57)

* change license to LGPL-3.0-only

* add license and CI changes to changelog

* use circleci for CI tests

* small docutils compatibility fix

* remove files that are build for docs

* minor fix for tests on windows

* use app.add_css_file instead of html_context

* fix broken links

* add missing docs badge

prepare for v0.1.0

as this speeds up starting the GUI for whatever reasons.

Add a reload button

Close the QApplication when closing the app widget

This commit adds a new subclass of the DatasetWidget
that lets the QApplication quit when
the window is closed.
This subclass is now used for starting the
app via command line

Gridlayout