Support `zarr` version 3

[david-zwicker]

Add support for zarr package version 3. It would be best if we could still support version 2, too (for the time being).

[david-zwicker]

Some migration guide is available here: https://zarr.readthedocs.io/en/stable/user-guide/v3_migration.html#migrating-to-zarr-python-3.

It seems that currently a number of features we needed are not supported by version 3 (see https://zarr.readthedocs.io/en/stable/user-guide/v3_migration.html#work-in-progress). In particular, structured data types, strings, and object data types are not implemented.