Ncvue

* Added conda and continuous integration badges.
* Allow multiple netcdf files.
* Squeeze output in `get_slice_miss` only if more than 1 dimension.

* Updated versions of github actions.
* Changed to sphinx_book_theme for documentation.
* Use local copy of `tooltip.py` from idle.
* Allow groups in netcdf files.
* Made ``ncvue`` work with newer matplotlib versions, updating
colormaps and using matplotlib.pyplot.style 'seaborn-v0_8-dark'.
* Made ``ncvue`` work with newer Tcl/Tk versions (ttk.Style.theme_use).

* Made ``ncvue`` a gui_script entry_point, so it can be called by
`python -m ncvue`.
* Bumped minimum Python version to 3.7 because of proj4.

* Added package_data to `setup.cfg`.

* Add final routines `add_cyclic` and `has_cyclic` committed to cartopy
v0.20.1.
* Added `ncvue` to conda-forge.
* Added `scripts` in [options] section in `setup.cfg`.

* Move to new pip structure using `pyproject.toml`.
* Versioning with `setuptools_scm`.
* Move to src directory structure.
* Move to Github actions.