Use special unit conversion for derived variables

[bouweandela]

Description

Closes ESMValGroup/ESMValTool#3709

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.62%. Comparing base (d472381) to head (3f3bf19).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2471   +/-   ##
=======================================
  Coverage   94.62%   94.62%           
=======================================
  Files         247      247           
  Lines       14055    14057    +2     
=======================================
+ Hits        13299    13301    +2     
  Misses        756      756           

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[schlunma]

Looks good, just a tiny comment. Thanks 🚀

[schlunma]

Successfully tests this with recipe_preprocessor_derive_test.yml 👍