Formatting fixes, Background images and viewport control buttons

Changed

• migrate to psyplot-plugin-template, see [!64 _
• set minimum version for psyplot to 1.5.0

Added

• Background image options, see [!65 _
• add buttons to control the viewport, , see [!66 _

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
• 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

Changed

• The plotmethod tabs have now a more intuitive gridlayout (see #46)
• When closing the mainwindow of psy-view now, one closes all open windows (i.e. also the open figures, see #47)

Added

• A widget to control the plot type for mapplot and plot2d (see #46)
• A button to reload all plots. This is useful, for instance, if the data on your disk changed and you just want to update the plot #48)

psy-view defines a viewer application for netCDF files, that is highly motivated by the ncview software, but entirely built upon the psyplot framework. It supports structured and unstructured grids and provides an intuitive graphical user interface to quickly dive into the data inside a netCDF file.