Include nested/flat tests

[joshmoore]

see: https://github.com/zarr-developers/zarr-python/issues/707

Most implementations support the choice of . or / as a dimension separator. The arguments for readers and writers could be: default, ., /. Auto-detection would be a potential other option depending on what is decided on 707.

Note: this may be the beginning of introducing many other dimensions to the test matrix.

[grlee77]

I started looking at this.

zarrita: can generate either flat or nested

zarr-python: can generate v2 files with nested format when using FSStore instead of the default DirectoryStore.

xtensor-zarr: it looks like it is possible to generate flat or nested for v3, but in the v2 spec case the '.' separator is currently hard-coded.

The n5 format does not seem to support a flat storage scheme

[joshmoore]

Sounds about right. A few notes:

• The n5 Java implementation does allow a choice of schemes, so we could choose to add support to the Python implementation. I haven't done so yet.
• It's still poorly tested, but https://github.com/zarr-developers/zarr-python/pull/716 should allow more directory stores to use the new metadata. Not sure if we want to use this repo to test that work, or settle that work first.
• Perhaps @davidbrochart would be up for adding it to v2 as well based on https://github.com/zarr-developers/zarr-python/pull/715?

[davidbrochart]

Perhaps @davidbrochart would be up for adding it to v2 as well based on zarr-developers/zarr-python#715?

Sure, that should be easy.

[joshmoore]

https://github.com/zarr-developers/zarr-python/pull/716 is now merged and released. I'd assume for all participating implementations, the behavior shoudl be:

• writer uses default --> .zarray with no dimension_separator --> reader uses "."
• writer uses "." --> .zarray with dimension_separator="." --> reader uses "."
• writer uses "/" --> .zarray with dimension_separator="/" --> reader uses "/"

and in all three cases something other than fill_value should be returned.