Finish NetCDF loading chunk control code

[trexfeathers]

(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)

[trexfeathers]

Make sure the concepts don't preclude later support for Zarr.

Other formats - PP, FF, GRIB - don't apply to this because the chunking is effectively set by the file structure.

[ESadek-MO]

Decisions on modes:

default: uses iris default behaviour (get from the file, if not, use iris guessing behaviour)
file: takes from the file, else throws an error if file doesn't specify.
guess: uses iris guessing behaviour only. (ignores file specifications, even if it exists)
custom (inc -1): Can manually input custom chunks. -1 might be inheritable from dask?

The use of "none" or "auto" should result in a warning or error to avoid dask confusion.