Ncvue

* Colorbar menu with images of colorbars.
* Use unlimited dimension instead of first dimension in Scatter and Contour
panels as default for 'all'.
* Use slice function rather than numpy.take to extract slices of arrays,
i.e. reads only the current slice from disk.
* Enhanced documentation with automatic API generation.

* Use zmin/zmax to fix colorbar for different dimensions in Contour panel.
* Optional grid lines in Contour panel.
* Possibility to invert x-axis in Scatter/Line panel.
* Rename Scatter to Scatter/Line.
* Use build instead of cibuildwheel to make pure Python wheels.

* Put common methods in ncvmethods (first arg is self).
* Make lists of labels, spinboxes and values of dimensions rather than
exhaustive lists of if/elif statements.

* Modularised ncvue to have utilities and different panels in individual
files.
* Open new window without helper class ncvWin to avoid circular import when
modularised.

* Initial release on Github.
* Scatter and Contour plot panels.