Clean up warnings coming from Iris in logs

[lewisblake]

When running the obs-only experiment I noticed the logs are very populated with repeteive warnings from Iris:

 warnings.warn(message, category=FutureWarning)
/modules/rhel8/user-apps/fou-kl/aerotools/pya-v2025.01/lib/python3.12/site-packages/iris/fileformats/netcdf/saver.py:2668: IrisDeprecation: Saving to netcdf with legacy-style attribute handling for backwards compatibility.
This mode is deprecated since Iris 3.8, and will eventually be removed.
Please consider enabling the new split-attributes handling mode, by setting 'iris.FUTURE.save_split_attrs = True'.
  warn_deprecated(message)

/modules/rhel8/user-apps/fou-kl/aerotools/pya-v2025.01/lib/python3.12/site-packages/iris/common/mixin.py:195: FutureWarning: You are using legacy date precision for Iris units - max precision is seconds. In future, Iris will use microsecond precision - available since cf-units version 3.3 - which may affect core behaviour. To opt-in to the new behaviour, set `iris.FUTURE.date_microseconds = True`.
  warnings.warn(message, category=FutureWarning)

These issues should be addressed to reduce these warnings