Filter non-spatial variables from UGRID datasets

[philipc2]

Discussed in #1324

^{Originally posted by Chrismarsh July 24, 2025}
What is the approach to save a subset ugrid to disk? For example, suppose I do

# massive dataset
ds = ux.open_mfdataset('output.2017.partial/output.2017.nc',
                       'output.2017.partial/output.2017.nc', 
                       parallel=False, lock=False)

# subset part of it
elev = ds.Elevation.subset.bounding_box(lon, lat)

# save subset
elev.to_xarray().to_netcdf('test2.nc', engine="netcdf4")

The test2.nc file does not contain valid ugrid topology:

$ ncdump -h test2.nc
netcdf test2 {
dimensions:
        n_face = 142546 ;
variables:
        double Mesh2_face_x(n_face) ;
                Mesh2_face_x:_FillValue = NaN ;
                Mesh2_face_x:standard_name = "latitude" ;
                Mesh2_face_x:long_name = "Characteristics latitude of 2D mesh triangle (e.g. circumcenter coordinate)." ;
                Mesh2_face_x:units = "degrees_north" ;
        double Mesh2_face_y(n_face) ;
                Mesh2_face_y:_FillValue = NaN ;
                Mesh2_face_y:standard_name = "latitude" ;
                Mesh2_face_y:long_name = "Characteristics latitude of 2D mesh triangle (e.g. circumcenter coordinate)." ;
                Mesh2_face_y:units = "degrees_north" ;
        double face_lon(n_face) ;
                face_lon:_FillValue = NaN ;
                face_lon:standard_name = "latitude" ;
                face_lon:long_name = "Characteristics latitude of 2D mesh triangle (e.g. circumcenter coordinate)." ;
                face_lon:units = "degrees_north" ;
        double face_lat(n_face) ;
                face_lat:_FillValue = NaN ;
                face_lat:standard_name = "latitude" ;
                face_lat:long_name = "Characteristics latitude of 2D mesh triangle (e.g. circumcenter coordinate)." ;
                face_lat:units = "degrees_north" ;
        double Elevation(n_face) ;
                Elevation:_FillValue = NaN ;
                Elevation:_QuantizeBitGroomNumberOfSignificantDigits = 4 ;
                Elevation:mesh = "Mesh2" ;
                Elevation:location = "face" ;
                Elevation:coordinates = "Mesh2_face_x Mesh2_face_y face_lat face_lon" ;
}

```</div>

[philipc2]

Related to #345