Zarr v3

[benjeffery]

I had a quick go at zarr v3 as the tskit CI broke due to tszip. It seems more involved than I expected, so will come back to it. Ideally we wish to support both v2 and v3, but this may not be easy.

[jeromekelleher]

Looking at how Tom is managing it in bio2zarr will help.

I would suggest letting the dust settle for a few months - I think a bunch of people are hitting incompatibility issues which will hopefully be resolved over the next while.