Make dims= optional for multi-group stores

[d33bs]

Today, a store with multiple dimension groups (e.g. GPCP) errors unless you pass dims=[...] (see src/read_zarr.rs:143). Two ways to reduce that friction, raised by @alxmrs in review of #29:

1. Guess the primary group so read_zarr(url) just works; keep dims= as an override.
2. Expose one table per dim group, with overridable names (like xarray-sql).

Already sketched in docs/design.md §"One table per dimension group" and §"ATTACH".