add table for versions numbers

[newville]

This adds a table for version numbers, so that potential changes to the schema might be handled, or auto-updated.

[auxym]

Hi, thanks for this. I am in the process of writing a draft spec document (quite simple) to describe the format, and I came to the same conclusion that we should include a table for sqlitestore-related metadata. I will open a PR soon to serve as an RFC for the spec.

[newville]

@auxym Great -- yeah, I think versioning would help allow multiple schema. Switching to somehing as complex as "Main is a Table of Groups", and "each Group is Tables of Arrays (and attributes)" and "Each Array as a table of chunks and attributes" would then be an implementation detail, and the code could probably support multiple schema. Support 'em all and see which one is the performance win for each use case.

I will say that I found the discussion over at Zarr-python to be worrying. I'm sure they mean well, but suggesting using S3 when asking about "single file" seems quite strange to me. And the "zarr uses key-value and that maps to Directory/Files" might be even weirder. Actually, key-value maps to Tables trivially. Yes, it could can be mapped to Directory/Files, but that is less obvious, and sort of requires convention/hidden assumptions which, as far as I can tell, are not versioned ;). I think the Zarr developers must be thinking about solutions to different problems.

I'm happy to try to help here, and am hopeful that this can be a great solution.