Add a function for finding input datasets for derived variables

[bouweandela]

Description

Alternative implementation to #2777 for finding input datasets for derived variables. Adds a method esmvalcore.dataset.Dataset.derived_variable_from_files that can be used to find input datasets.

Note that derived variables with optional input datasets are not yet supported. The only derived variable that uses this feature is amoc at the moment, so the impact of that is limited.

Link to documentation: https://esmvaltool--3051.org.readthedocs.build/projects/ESMValCore/en/3051/notebooks/derived-variables.html

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Checklist

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• 🧪 The new functionality is relevant and scientifically sound
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[codecov]

Codecov Report

❌ Patch coverage is 14.28571% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.08%. Comparing base (df77d1b) to head (87cce7e).

Files with missing lines	Patch %	Lines
esmvalcore/dataset.py	14.28%	12 Missing ⚠️

❌ Your patch check has failed because the patch coverage (14.28%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3051      +/-   ##
==========================================
- Coverage   96.15%   96.08%   -0.08%     
==========================================
  Files         270      270              
  Lines       15805    15819      +14     
==========================================
+ Hits        15198    15199       +1     
- Misses        607      620      +13     

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