Remote Dataset Explorer: gather information from directory listings

[fm3]

Some n5 or zarr datasets have no meta information about layers in the top level. We would have to traverse its directories until we find valid metadata. Example dataset: s3://janelia-cosem-datasets/jrc_hela-3/jrc_hela-3.n5 (where we would need to find the sub-path em/fibsem-uint16, which then contains valid attributes.json with multiscales)