multiscene: load zarr group as scenes

[toloudis]

Use Case

In the absence of a mature ome-zarr spec for collections, (see allen-cell-animated/vole-core#276) try to detect if a zarr url holds a group of zarr arrays.

Solution

Can we detect this scenario and do the right thing without user intervention? E.g. look at url, ask if it is a zarr group, if so, are the children actual zarr images? If so then treat this as a multiscene.