Support global scale transformations at the multiscale level

[tcompa]

This currently raises NotImplementedError's here and there in the tasks, but it should be supported. Note that it part of the OME-NGFF specs, but it's not supported by ome-zarr reader.

[tcompa]

References:
https://ngff.openmicroscopy.org/latest/#multiscale-md
ome/ome-zarr-py#172

[jluethi]

There was a bit of a discussion on this topic at today's OME-NGFF community call. Global scales should be allowed (so would make sense to eventually support them), but multiple people say they don't recommend doing that. There's a bit of ongoing discussion on how the 0.5 OME-NGFF spec will word this. So let's wait for that to be finalized before adding this support. Likely we will stay at what we're currently doing as a default, but may eventually implement global scales as well.