Default chunking has changed

[dougiesquire]

With xarray > v2023.09.0, the chunking behaviour of xarray's open_dataset has changed. Now:

• chunks="auto" returns variables with chunk sizes as multiples of the chunks in the nc file,
• chunks={} returns variables with the exact nc chunk sizes.

The chunking tutorial in the documentation does not reflect this, but should.

[dougiesquire]

See https://github.com/intake/intake-esm/issues/632