Add (commented) import and to_netcdf call in Python export

[Huite]

The current export doesn't produce any file output. There's obviously functionality for this in gistim, which would help people to write their output and visualize it in QGIS.

To avoid dependency trouble and failing imports, we could comment it out by default.

[Huite]

This should likely go into the documentation as a snippet instead.

One of the complications is communicating the CRS information. This may require a CRS string, which is very long...