BUG: ugriddatasetaccessor.to_geodataframe

[hboisgon]

I tried using the to_geodataframe method for UgridDataset but found two issues:

• sometimes, the Dataset can contain no variables yet (eg uds = Ugrid2d.to_datatset()) and this is not allowed
• the crs is not passed to the geodataframe and is lost in the conversion.

The lines in question:

https://github.com/Deltares/xugrid/blob/b6c4fb5570af34a264783e709e948114255a6a71/xugrid/core/dataset_accessor.py#L394-L403

An idea of implementation:

gdf = gpd.GeoDataFrame(
    index=grid.to_dataset()[dim].values.astype(str),
    data=ds[variables].to_dataframe(dim_order=dim_order) if len(variables)>0 else None,
    geometry=grid.to_shapely(dim),
    crs = grid.crs,
)

[Huite]

Hi @hboisgon,

The CRS thing has an issue for itself with #138. Allowing to create a GeoDataFrame with only geometry seems reasonable.