Multi-level datasets are Zarr groups

[forman]

This PR allows multi-level datasets to be regular Zarr groups so we can open them using xr.open_datatree().

Closes #1123

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)