update for zarr 3

[peterdudfield]

Update the code for zarr3

We have to work out if the zarr file is 2 or 3, and then use the correct driver when looking at the data

#20

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[shoyer]

Thanks Peter! A colleagues ended up noticing the same thing and put together a very similar fix which we just merged in #18

I'll go ahead and make a new release shortly.