It would be very nice to be able to read, with `cf.read`, [Zarr](https://zarr-specs.readthedocs.io) datasets that contain a group hierarchy. #863 implemented the reading of Zarr datasets, but only those with no group structure (i.e. only the root group). Zarr v2 (xarray-style) and Zarr v3 will be supported. This will almost entirely implemented in `cfdm` (https://github.com/NCAS-CMS/cfdm/issues/355), and inherited from there.