xugrid.open_dataset results in a locked netCDF file

[Huite]

Current implementation does this:

def open_dataset(*args, **kwargs):
    ds = xr.open_dataset(*args, **kwargs)
    return UgridDataset(ds) 

Internally, the UgridDataset constructor creates a new dataset, and so the ds reference is lost, making UgridDataset.close() a dud.

We should likely do something like:

class UgridDataset:
    def __init__(self, dataset):
        self._original = dataset
        # ... rest of initialization
    
    def close(self):
        self._original.close()

It might be possible that dataset is None, though.

Explicit indexes would also solve this, since we'd just use/return an ordinary xarray dataset.