Move towards zarr multi-scale spec

[manzt]

Our current approach outputs a directory structure for tiled image pyramids as:

└── example/
    ├── .zgroup
    ├── base
    │   ├── .zarray
    │   ├── .zattrs
    │   ├── 0.0.0
    │   └── ...etc
    └── sub-resolutions/
        ├── .zgroup
        ├── 01/
        │   ├── .zarray
        │   ├── 0.0.0
        │   └── ...etc
        └── 02/
            ├── .zarray
            ├── 0.0.0
            └── ...etc

In zarr-developers/zarr-specs#50, a spec has been been proposed to define the group/array structure as well as metadata in the group:

example/
├── 0    # Full-sized array
├── 1    # Scaled down 0, e.g. 0.5; for images, in the X&Y dimensions
├── 2    # Scaled down 1, ...
├── 3    # Scaled down 2, ...
└── 4    # Etc.

{
  “multiscale”: {
    “version” : “0.1”,
    “series” : [
      {
        “datasets” : [“0”, “1”, “2”, “3”, "4"]
      }
     // See link for more details
  }
}

While I think having a separate group for "sub-resolutions" makes sense, since an image might be stored in in full resolution as zarr independent of sub-resolutions (and complies with the OME-spec), but I think we should target what is decided here.