Use our own unit conversion function in our fixes

[schlunma]

Description

While performing tests for ESMValGroup/ESMValTool#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

• Add unit conversion to DU to recipes that use toz ESMValTool#3784
• Make derivation of total column ozone (toz) more flexible and add derivation of stratospheric and tropospheric column ozone #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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.79%. Comparing base (f622e40) to head (5968487).

Additional details and impacted files

@@            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]

thanks Manu 🐊 A few questions I have, unfortunately all packed into a comment 🍺

[valeriupredoi]

spiffy! Still, no alligators 😢 🐊