Ncvue

* Better installation instructions for all platforms.
* Dropped claim of Python 2 support, which was not given.

* Main window disappears if closed even if called from within Python.
* Added Windows Installer for ncvue standalone program.
* Added standalone app for macOS.

* Added 'Open File' button to switch between files.
* Separated Tk() and Toplevel() to communicate via Tk() between windows.
* Externalise analysis of netcdf file for open file button.
* Allow finding images path in standalone applications using pyinstaller.

* Include `ncvue/images/*.png` in PyPI wheel.

* Moved from ReadTheDocs to Github Pages for documentation.
* Added tooltips to all selectors, entries, menus.
* Variable names are now separated by SEPCHAR=chr(6) because netcdf variable
names can have spaces, parentheses, brackets, etc.
* Map panel is only chosen first if either lon or lat have more than one
grid cell.
* Central longitude is now calculated in 0-360 range but set in -180 to 180
range. Seems to be more stable for grids that are missing for example the
southern hemisphere.
* Catch a few errors if variable is for example a simple string (e.g.
vegetation type, basin name or similar).

* Added Map panel.
* Assure 2-digit month and day and 4 digit year in time unit.
* Added return on numeric keyboard to key bindings.
* Changed layout so that dimensions are below variable selection spinboxes.
* Transpose array by default in Contour panel so that first dimension (time)
is on x-axis (col) because contourf/pcolormesh use (row,col).
* Common arithmetic operations on axes: mean, std, min, max, ptp, sum,
median, var.
* General get_slice function for x, y, y2, and z.
* Only activate valid dimensions for chosen variable, disable others.
* Axis labels are now the long_name attribute then the standard_name
attribute and only if both are missing the variable name.