m300977: Added example_notbooks.rst to list notebooks in a reasonable order for documentation. Linking the notebooks into the documentation does not work so far.

Added notebooks/pyicon_view_Apollo.ipynb to demonstrate how to visualize R2B11 data with pyicon.view.

pyicon/view/pyicon_view_xr.py: Try to identify grid first by uuidOfHGrid, allow for specifying coordinates to enable plotting vertex variables, allow for plev (pressure levels) dimension.