Zarr v3: support root path

[grlee77]

closes #1039

v3 spec states path = '/' for arrays gives an array at /meta/root.array.json path = '/' for groups gives a group at /meta/root.array.json

In this implementation path=None will also result in a root array. Creation routines default to path=None, so this makes it so that the path argument does not have to be manually specified.

TODO:

• [x] 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)

[codecov[bot]]

Codecov Report

Merging #1085 (e8c23c8) into main (ece1810) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1085   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files          34       34           
  Lines       13846    13853    +7     
=======================================
+ Hits        13839    13846    +7     
  Misses          7        7           

Impacted Files	Coverage Δ
zarr/hierarchy.py	99.78% <ø> (-0.01%)	:arrow_down:
zarr/tests/test_convenience.py	100.00% <ø> (ø)
zarr/_storage/store.py	100.00% <100.00%> (ø)
zarr/creation.py	100.00% <100.00%> (ø)
zarr/storage.py	100.00% <100.00%> (ø)
zarr/tests/test_core.py	100.00% <100.00%> (ø)
zarr/tests/test_creation.py	100.00% <100.00%> (ø)
zarr/tests/test_hierarchy.py	100.00% <100.00%> (ø)

[joshmoore]

@grlee77, thanks for this! Reading over it, it does seem to address the issues that Stephan raised. Do you have anything else for this PR from your side or just need testing and then rolling into a 2.12.x release??

[grlee77]

I think test coverage is pretty good now. It should be ready for review.

[joshmoore]

Rolling into a 2.13 pre-release for better testing together with xarray.

cc: @shoyer