Output: add '_FillValue' alongside 'missing_value' in nc output

Closes #159 (closed)

We never added the attribute _FillValue to the netcdf output files.

_FillValue is used to fill data arrays that were not yet written and is used to indicate missing data ("missing_value" is more meant for documentation):

• https://docs.unidata.ucar.edu/netcdf-c/current/attribute_conventions.html
• https://github.com/Unidata/netcdf4-python/issues/632