Add decompressed OME-Zarr dataset size to iohub info

[edyoshikun]

This addresses issue #247 by adding the store size and array size in GB. This is useful and simple metadata.

I wanted to know how much memory to request for caching datasets.

[ziw-liu]

Is this meant to represent the size on disk (compressed) or size in RAM (decompressed)?

[edyoshikun]

I find it more use when it's decompressed rather than compressed. We can report both if needed. I think zarr.array does nbytes_stored. What do you guys think?

[talonchandler]

I think the uncompressed size is the most valuable.

The reported size is the expected size, not the true size (e.g. it hasn't been filled yet or there was an error). Naming is tricky---maybe "Expected uncompressed size (GB)", "Est. size in RAM (GB)", or "Est. size (GB)"?

[edyoshikun]

ended up adding uncompressed size [GB]

[ziw-liu]

Need to add a test case before merging.

[ziw-liu]

Due to upstream issue zarr-developers/zarr-python#2174, nbytes_stored will be wrong for OME-Zarr. I think we should just remove this field since the zarr devs are probably not focusing on v2 bugs now.

[ziw-liu]

For example this compression ratio is clearly wrong:

No. bytes:               88473600 [84.4 MiB]
No. bytes stored:        419 [419 B]