Multi-level datasets should be Zarr groups

[forman]

Is your feature request related to a problem? Please describe.

It would be handy if xcube multi-level datasets could be opened using xrarray.open_datatree().
A missing .zgroup is currently preventing this.

If we add a Zarr v2 .zgroup:

we can open them:

Describe the solution you'd like

1. always write .zgroup when writing multi-level datasets and optionally generated consolidated metadata for group.
2. Move contents of .zlevels into .zattrs. (pros and cons TBD)
3. Update the dataset spec to reflect this.