V3 reorg

[jhamman]

This PR moves the v3 source code to the project root and preserves the v2 sources in a new v2 directory.

Process taken:
Step 1: move all v2 sources and tests to v2 directories, update imports and tests, commit -> ee09dec
Stepo 2: move all v3 sources from v3 directory to zarr root, update imports, tests, docs, and pyproject.toml, commit -> 51780ed

TODO:

• 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)

[jhamman]

this is showing up as a line-edit diff but no edits were made. I first moved the original __init__.py -> v2/__init__.py, then made a commit. Then the v3/__init__.py file was moved here in a second commit.

[jhamman]

this is an actual change, although not a very consequential one.

[d-v-b]

I didn't check every change, but the general direction looks good and if there's any rough edges we can iron those out later.

[joshmoore]

A pretty uninformed question, please take it with a grain of salt, but would keeping the main directories and aliasing into the main namespace be more forward compatible? (i.e., avoid the need to move everything back to v3 when/if v4 comes along)

[normanrz]

A pretty uninformed question, please take it with a grain of salt, but would keeping the main directories and aliasing into the main namespace be more forward compatible? (i.e., avoid the need to move everything back to v3 when/if v4 comes along)

Imo, the v2 directory should be gone before we release. We're just keeping it for now to easier move code around. I don't think it would be wise to keep essentially 2 code bases to maintain.