OME-Zarr 0.5: add version to well group attributes

[sbesson]

Discovered while generating a set of OME-Zarr 0.5 datasets with 0.12.0-rc3, the metadata written for the well group is currently invalid as it is missing a version attribute - see e..g https://ome.github.io/ome-ngff-validator/?source=https://gs-public-zarr-dev.s3.amazonaws.com/zarrv3/NIRHTa%252B001.zarr/A/1/

This PR fixes this by:

• populating the attribute storing to include the version as for the other groups
• extending the HCS tests to check the well group attributes: version and well/images

The README is also expanded to cover the multi-version support and the --ngff-version option. As this is slightly outside the scope of the initial problem, happy to move the last commit to a dedicated PR if that is preferred.

[erindiel]

Confirmed with conversion of https://downloads.openmicroscopy.org/images/PerkinElmer-Operetta/idr0034/ with --ngff-version 0.5 that the zarr.json in each well (.zarr/<letter>/<number>/zarr.json) contained a version key:

{
  "zarr_format" : 3,
  "node_type" : "group",
  "attributes" : {
    "ome" : {
      "well" : {
        "images" : [ {
          "path" : "0",
          "acquisition" : 0
        }, {
          "path" : "1",
          "acquisition" : 0
        }, {
          "path" : "2",
          "acquisition" : 0
        }, {
          "path" : "3",
          "acquisition" : 0
        }, {
          "path" : "4",
          "acquisition" : 0
        }, {
          "path" : "5",
          "acquisition" : 0
        }, {
          "path" : "6",
          "acquisition" : 0
        }, {
          "path" : "7",
          "acquisition" : 0
        }, {
          "path" : "8",
          "acquisition" : 0
        } ]
      },
      "version" : "0.5"
    }
  }
}

I have a small note on the README, but it isn't required for this to be merged.

[erindiel]

I was considering changing this to may result and may not strictly follow, but I think instead the note in --ngff-version section is better.

[erindiel]

Suggested change

	Specifies the version of the [OME-Zarr specification](https://ngff.openmicroscopy.org/specifications/index.html)
	that should be used while writing Zarr. Current supported values are 0.4 and 0.5.
	Specifies the version of the [OME-Zarr specification](https://ngff.openmicroscopy.org/specifications/index.html)
	that should be used while writing Zarr. Current supported values are 0.4 and 0.5.
	Both options produce outputs which are compatible with raw2ometiff.

Are we comfortable saying this? Assuming both will be released together...

[sbesson]

I think this is correct if no additional option is passed but as noted above if additional formatting options are passed, the Zarr dataset might be stored using a layout that makes it incompatible with raw2ometiff or other OME-Zarr aware tooling

[melissalinkert]

Merging as discussed with @Yajing826 and @sbesson so that we can unblock adding tests to #317. Propose that we do a final review of the README next week prior to tagging 0.12.0 to catch any final documentation issues.