Network file writer does not write correct conventions

[priscavdsluis]

Describe the bug
When exporting a network file via HYDROLIB-core it writes conventions that the GUI complains about.

Please investigate what is needed here.

[RuudHurkmans]

When I change the conventions in a postprocessing-step (opening the created file in netCDF4) like this:

conventions = 'CF-1.8 UGRID-1.0 Deltares-0.10'

The warning in the GUI disappears.

As a related topic, there is a warning about a missing projection when trying to run a model:

** WARNING: Missing coordinate reference system. Now using default: projected_coordinate_system (EPSG:0).

I have been trying to add EPSG:28992 to the net-file but the warning persists. Is there information about how the GUI expects this information?