Bump to 0.2: use nested chunks

[joshmoore]

see: ome/ngff#40

[will-moore]

I don't know how long all the various version numbers will stay in sync, but for now it would be easier if there was a single VERSION variable to use in all these places so we don't miss one.

[will-moore]

This PR seems to be causing directories in a Plate to be named with lowercase.
Also, the .zattrs and .zgroup have the . missing from their names:

E.g. $ omero zarr export Plate:251

Without this PR:

$ ls 251.zarr/
A	B	C

With this PR:

$ ls 251.zarr/
a	b	c	zattrs	zgroup

[joshmoore]

@will-moore : it's unlikely I'll be getting to this PR today if you want to push any commits.

[will-moore]

Ah, OK the naming of .zattrs .group is fixed by upgrading zarr to zarr-2.7.0.
But still seeing lowercase dir names...

[will-moore]

Exporting with this current PR: Data visible at https://deploy-preview-85--vizarr.netlify.app/?source=https://minio-dev.openmicroscopy.org/idr/idr0001-graml-sysgro/pr59_nested/2551.zarr (from PR hms-dbmi/vizarr#85)