Added Group.tree method

[TomAugspurger]

Adds a Group.tree method for pretty printing a hierarchy.

For now the old expand keyword, which I think controlled whether the tree was expanded by default in the HTML output, is not implemented.

We have one design choice to make: should .tree return an object with a custom __repr__, or should it return the string itself? My preference, implemented here, is returning the object. That makes it easier to pipe into other applications other than the Python REPL. The cost is a bit more expansion of the public API.

Closes #2406

• 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/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[TomAugspurger]

There was a notebooks/repr_tree.ipynb notebook that I just ran through. This PR does not implement

• A plain ASCII tree repr with print(bytes(g1.tree()).decode()). We kind of get that for free through rich, which will automatically detect whether the console being rendered to supports unicode or just ASCII. We won't have an API for doing that (we can probably document how to configure rich to get out an ascii repr if someone really wants that)
• An interactive HTML repr (I think for collapsing nodes). I've toyed around with the idea of a textual-based TUI for Zarr groups that would be a better spot for this kind of interactive explorer IMO.

If we're OK with those limitations then this should be good to go.

[jhamman]

Looks great @TomAugspurger!

[jhamman]

cc @d-v-b who is looking at making this faster 🚀

[jhamman]

👍