(Complete the below work on a feature branch)

Documentation has been separated into another issue to make it easier to find time for the work. So chunk control will not be 'releaseable' when this issue is complete.

• Create a new PR applying Dask chunking control for netcdf loading. #4572 to the latest Iris code (this code has developed a lot so the original PR is too stale for updating)
  • (COMPLETED: Merge chunk control code into latest iris #5565)
• Action original conversations Dask chunking control for netcdf loading. #4572 (comment) may need more discussion; suggest that mirroring Dask 'modes' should only happen if the behaviour is identical, otherwise the modes deserve their own names ("default" or "no-chunk" or whatever) to avoid possible confusion.
  • (COMPLETED: Chunk control modes #5575)
• Flesh out the tests.
  • (COMPLETED: Chunk Control Tests #5583)