Backed multiscale image, problems with scales and dims

[LucaMarconato]

Possible bug with scales

Bug originally reported here from @aeisenbarth in the context of another issue: https://github.com/scverse/spatialdata/pull/348#issuecomment-1702672491.

Rasterschema.validate(MultiscaleSpatialImage) expects all scale levels to have a transform, but the backed image has only the transform in scale0.

Possible bug with dims

Bug originally reported here from @aeisenbarth in the context of another issue: https://github.com/scverse/spatialdata/issues/347

For backed labels, xdata is not a DataArray (where dims is ("y", "x")) but a DataTree where dims is Frozen({"y": 1633, "x": 1290}). See more details in the linked issue in points 2. and 3.