Add support for delayed saves

[bouweandela]

Since iris 3.6, iris supports delayed saves. This is a great new feature that allows:

• checking that a preprocessor run will succeed before doing any data crunching, saving (compute) time if there is something wrong with the data
• faster preprocessor runs because
  • intermediate results can be reused to write multiple output files (e.g. regrid only once for multimodel statistics)
  • I suspect this will allow to better overlap computations with writing/reading data, but I did not measure it so far.

See #1714 for an example implementation.

[bouweandela]

Dropping this from the v2.10 milestone because there is no more time. Some funding has been allocated in ESiWACE3, so there may be time to work on this for v2.11 or v2.12.