Read tables using anndata._io.read_zarr for local data

[will-moore]

This creates a Node for each table listed under image.zarr/tables/.zattrs and adds a node.table AnnData object to the node. The node.metadata dict is populated with the image.zarr/tables/my_table/.zattrs data.

These tables can be used by e.g. napari-ome-zarr as needed to read "regions" or "tracking" data etc.

We read remote anndata using anndata.experimental methods so the anndata requirement is pinned to an exact version.

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.64 ⚠️

Comparison is base (9540832) 84.54% compared to head (17414e3) 83.91%.

❗ Current head 17414e3 differs from pull request most recent head 29eb30c. Consider uploading reports for the commit 29eb30c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   84.54%   83.91%   -0.64%     
==========================================
  Files          13       13              
  Lines        1475     1498      +23     
==========================================
+ Hits         1247     1257      +10     
- Misses        228      241      +13     

Impacted Files	Coverage Δ
ome_zarr/reader.py	83.91% <50.00%> (-2.15%)	⬇️

... and 2 files with indirect coverage changes

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[will-moore]

@joshmoore/@sbesson If this approach looks good to you, I'll start to add tests?
The only bit of metadata that isn't easily accessible by API is the "name" of the table, e.g. image.zarr/tables/table_name/ which is available at the end of the node's path. I could add to this to the node.metadata, which would "pollute" the anndata metadata: e.g.

{
    "table-name": "table_name",
    "encoding-type": "anndata",
    "encoding-version": "0.1.0",
    "type": "ngff:points_table"
}

but I'll probably hold off for now, since it's not needed, and can be got from path.

[will-moore]

Closing since tables spec is not changing just now...