Rename "masks/" to "labels/"

[joshmoore]

@sbesson points out that since we decided to move to the "labeled-image" naming/structure, the use of "masks/" as a group name is now potentially confusing. "labels/" is likely as good and will give us a chance to test a breaking change from v0.1 to v0.2!

[joshmoore]

PRs opened to unify on "labels". Note this is a breaking change but such data was not fully released so I'm unsure whether or not to treat it as such. With a little work, most of the code bases could be taught to handle both versions so that both 0.0.12 and the next version of ome-zarr-py could be handled.

It may be easier to simply update the datasets to have both for the moment, and then delete masks/ at some point in the future. These datasets need updating:

6001237.zarr/
6001238.zarr/
6001239.zarr/
6001240.zarr/
6001241.zarr/
6001242.zarr/
6001243.zarr/
6001244.zarr/
6001245.zarr/
6001246.zarr/
6001247.zarr/
6001248.zarr/
6001249.zarr/
6001250.zarr/
6001251.zarr/
6001252.zarr/
6001254.zarr/
6001255.zarr/
6001256.zarr/
6001258.zarr/
9836950.zarr/

cc: @dominikl @will-moore @mtbc

[joshmoore]

Once the color decision is made, this change should be applied to all public datasets.