Threedigrid

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- Added flod and flou attributes to lines for possible reading obstacle heights.

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- Added 'has_dem_averaged' attribute to cells.

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- Exporters now export NaN and -9999.0 float values as NULL.

- Fixed exporting string dtype fields (e.g. cont_type) in OGR exporter. For instance,
the string "b'something'" now is written as "something".

- Skip the dummy element (with id=0) in all exporters.

- Set the FID (feature ID) in the OGR (shapefile/geopackage/some geojson) exporters.

- Deprecate specific serializers.py under threedigrid.admin.breaches, .lines, .nodes and
.pumps.

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- Fix GeoJSON levees coordinate order.

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- Renamed the 's_1d' field under lines to 'ds1d_half'.

- Added 'initial_waterlevel' to nodes.

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- Added 'nodes_embedded', available under gridadmin Class.