wip new lazy to_multiscale() for labels

[LucaMarconato]

Closes #1069

This PR (still work-in-progress) aims at implementing a function to generate a pyramidal image given a dask array, by relying on ome-zarr internal APIs and by extending functionalities currently not available via ome_zarr.scale.Scaler. Such a function is also provided by the multiscale-spatial-image package, but as explained in the linked issue we hit a performance roadblock.

ome-zarr-py does not support downscaling of the z dimension, nor downscaling via scale factors specified per dimension (both cases are supported by multiscale-spatial-image); these cases will be covered by this PR. The implementation could also live in ome-zarr-py (CC @will-moore @joshmoore @jo-mueller), I'd be happy if someone later can pick up the code so I can remove it from spatialdata and use it from upstream.

Side technical note:

• the previous implementation in spatialdata uses xarray_coarsen (from multiscale-spatial-image) for images, (which is fully lazy), and itk_nearest (from multiscale-spatial-image) for labels (which is not lazy and therefore problematic). Since downscaling for images doesn't give problems, we could use the old implementation, but I'm reimplementing it with ome-zarr-py APIs anyway. So far it seems that both implementations lead to identical results.

[jo-mueller]

Hi @LucaMarconato ,

PR ome/ome-zarr-py#516 also addresses the issues with the downscaling in z (now built in for all methods) 👍