Global storage transformers

[rabernat]

As discussed in the ZEP call last week, this is a first pass at creating a "global storage transformers" concept, to be applied at the store (rather than array) level.

There are a few things that need cleaning up here:

• Double check that all RST references point to the right place
• Add more detail and example use-cases of both global and array storage transformers
• Add an example of a global storage transformer in zarr.json

But before doing that, I thought I would solicit some feedback on this. Does anyone see any fundamental problems with this concept? My motivating use case is #154.

My main open question is: when both array and global storage transformers are defined, what determines the order in which they are applied? The only feasible choices appear to be to apply all the global transformers either before or after the array transformers. Should we make this an option in the protocol?

[jbms]

I think array storage transformers should definitely apply before global storage transformers, since the array metadata itself is obtained through the global storage transformers.

In fact though it might make sense to also support group-level storage transformers, which would stack with the nesting of groups on top of any global storage transformers.

[jstriebel]

Depends on #192

[joshmoore]

Now conflicting.

[jstriebel]

@rabernat Could you adapt this PR to cover group storage transformers for the current spec?

[rabernat]

On today's call, we decided that we would defer this to an extension after the core v3 spec has been finalized.