Owen Littlejohns

@TomNicholas - I think this PR is at a point now where it can be reviewed again in earnest. 🤞

> open_datatree in xarray. This doesn't need to be performant initially, and it would initially return a datatree.DataTree object. So we can start by just copying the basic version in...

I made the change requested above (updating the import for `collections.abc.Iterable`). I also rebased the branch to be on top of the most recent version of the `master` branch. If...

For reference, my test case: ``` dataset = open_url('dap4://opendap.earthdata.nasa.gov/collections/C1996881146-POCLOUD/granules/20230511090000-JPL-L4_GHRSST-SSTfnd-MUR-GLOB-v02.0-fv04.1', timeout=1) ```

Okay. I've merged this and I'll get cracking on the next `pydap` release to PyPI. Thanks, @jgliss for the fix!

Thanks @kandersolar - yes, we now have [a GitHub workflow](https://github.com/pydap/pydap/blob/master/.github/workflows/publish_to_pypi.yml) that allows us to easily publish to PyPI. It triggers either on the publication of a new GitHub release, or...

@TomNicholas - So the alternative here is exposing things in the public API, then updating this PR to have the correct references and merging this afterwards? (Just checking I read...

I have added `open_datatree`, `DataTree`, `register_datatree_accessor`, `map_over_subtree` and `assert_isomorphic` to the public API. The [DataTree public API](https://github.com/xarray-contrib/datatree/blob/main/datatree/__init__.py) has some other exceptions in it (`TreeIsomorphismError`, `InvalidTreeError` and `NotFoundInTreeError`), but I was...

@TomNicholas - to clarify your previous comment about those exceptions: would you like those 3 exceptions added to the public `xarray` API (`TreeIsomorphismError`, `InvalidTreeError`, `NotFoundInTreeError`)? Or would you just want...

Thanks for the guidance. [I just added those three exceptions to the public API](https://github.com/pydata/xarray/pull/9033/commits/02287770d38660bf845c485d652d5d8bc4cd83db), so they are now consistent with `MergeError`.