Use our own unit conversion function in our fixes

[schlunma]

Description

While performing tests for https://github.com/ESMValGroup/ESMValTool/pull/3784, I noticed that our fixes do not use our own unit conversion function, which allows special unit conversions like m -> DU.

This PR adds this, but uses a "safe" version of the unit conversion which will not allow a change of the cube's standard_name (this would lead to errors in the subsequent CMOR checks).

Note: I had to restructure the functions a little bit to avoid circular imports.

Related to

• https://github.com/ESMValGroup/ESMValTool/pull/3784
• https://github.com/ESMValGroup/ESMValCore/pull/2509

Link to documentation:

• https://esmvaltool--2560.org.readthedocs.build/projects/ESMValCore/en/2560/api/esmvalcore.iris_helpers.html#esmvalcore.iris_helpers.safe_convert_units
• https://esmvaltool--2560.org.readthedocs.build/projects/ESMValCore/en/2560/api/esmvalcore.preprocessor.html#esmvalcore.preprocessor.convert_units

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Checklist

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• [x] 🛠 Unit tests have been added
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.79%. Comparing base (f622e40) to head (5968487). Report is 9 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2560 +/- ## ========================================== + Coverage 94.77% 94.79% +0.02% ========================================== Files 251 251 Lines 14266 14280 +14 ========================================== + Hits 13520 13537 +17 + Misses 746 743 -3 ```

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[valeriupredoi]

@ESMValGroup/technical-lead-development-team could one of yous please have a last look at this then merge? :beer: