Final touches for a stable release of iohub v0.3.0.

[srivarra]

The discussion follows: czbiohub-sf/biahub#199 (comment)

what is needed to release a stable v0.3? Which of the issues on this milestone are critical, and which can be pushed to a v3.1 milestone?

After looking at the issues left for this milestone, I think all of them can be pushed back to >=v0.3.1. The only thing I'd like to get fixed would be: #374.

For the shard / chunk sizes currently:

• we have set to 500 mb in for ZYX... for the conversion step
  • Should _limit_zyx_chunk_size be set to 5mb then?
• For sharding, should we set some defaults via the convert to allow for (1, 1, Z, Y, X) during processing?