Yet another alternative implementation for working with derived variables

[bouweandela]

Description

Yet another alternative implementation to #2777 and #3051 for finding input datasets for derived variables and loading them. Adds a new class method `esmvalcore.dataset.DerivedDataset that can be used to work with derived variables.

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--3053.org.readthedocs.build/projects/ESMValCore/en/3053/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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• 🛠 Code is written according to the code quality guidelines
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• 🛠 Unit tests have been added
• 🛠 Changes are backward compatible
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[codecov]

Codecov Report

❌ Patch coverage is 37.50000% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.00%. Comparing base (5ca0b1d) to head (de9351d).

Files with missing lines	Patch %	Lines
esmvalcore/dataset.py	37.50%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3053      +/-   ##
==========================================
- Coverage   96.15%   96.00%   -0.15%     
==========================================
  Files         270      270              
  Lines       15805    15844      +39     
==========================================
+ Hits        15197    15211      +14     
- Misses        608      633      +25     

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