Zarr chunking over non-spatial dimensions

In our current Zarr implementation after #445 , deck.gl-raster will select the _spatial dimensions_ but the user must provide a constant `selection` for the non-spatial dimensions.

In the future we should consider chunking over non-spatial dimensions. I'm not sure exactly what that would look like. Neuroglancer is a good reference

Start by reviewing the Zarr implementation changes from #445 and the current handling of spatial dimensions and constant non-spatial selection. Compare the relevant behavior with Neuroglancer, then define how chunking over non-spatial dimensions should work. Done means a concrete, agreed design and scope for the feature.

Written by the indexing model from the issue text.