zarr v2 to zarr v3 conversion

[ruaridhg]

Work out a way to do this ‘in-place’, ie. just by re-writing the metadata document and not copying any data.

Discussed zarr-developers/zarr-python#1798

There's an issue for zarr v2 > v3 conversion on zarr-python
Current implementation of conversion
Ask on issue about using current implementation.

[K-Meech]

I'll close this, as the PR has been merged into zarr-python: zarr-developers/zarr-python#3257 🎉