Use `iris.FUTURE.save_split_attrs = True` to remove iris warning in many diagnostics

[schlunma]

Description

Remove iris warnings similar to https://github.com/ESMValGroup/ESMValCore/pull/2398.

In this PR this is only done for the diagnostics I know well. If desired, I can add this to all of them, but there might be the potential of recipe failures.

---

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• [x] 🛠 This pull request has a descriptive title
• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🛠 Documentation is available
• [x] 🛠 Tests run successfully
• [x] 🛠 The list of authors is up to date
• [x] 🛠 Any changed dependencies have been added or removed correctly
• [x] 🛠 All checks below this pull request were successful

---

To help with the number of pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[schlunma]

even better, maybe move the iris imports and set it once in esmvaltool/diag_scripts/shared/__init__.py? If not, I think iris_helpers.py needs it too

It's probably clearer to have it explicitly in a file. As far as I can tell, iris_helpers.py does not have a iris.save call.

[schlunma]

I merge this now to make sure it is included in v2.11.0. I tested most affected recipes, and the output looks fine 👍

[valeriupredoi]

ah yeah sorry Manu - that's why I approved, am not too fussed about my optional suggestion, go for it! if you wanna see docs hell, have a look at the Autoapi PR I am working on, that's why I got distracted off this one :beer: Make sure to ping @chrisbillowsMO to be sure-sure this is included in the El Releaso

[schlunma]

No problem! There is no release branch in ESMValTool yet, so no need to cherry-pick anything 👍