Extend preprocessor anomalies

[axel-lauer]

Description

This PR extends the existing preprocessor anomalies to allow for optionally calculating relative anomalies (in percent).

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🧪 The new functionality is relevant and scientifically sound
• 🛠 This pull request has a descriptive title and labels
• 🛠 Code is written according to the code quality guidelines
• 🧪 and 🛠 Documentation is available
• 🛠 Unit tests have been added
• 🛠 Changes are backward compatible
• 🛠 Any changed dependencies have been added or removed correctly
• 🛠 The list of authors is up to date
• 🛠 All checks below this pull request were successful

[codecov]

Codecov Report

❌ Patch coverage is 31.25000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.41%. Comparing base (1ee7ae6) to head (3da2feb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
esmvalcore/preprocessor/_time.py	31.25%	11 Missing ⚠️

❌ Your patch check has failed because the patch coverage (31.25%) 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    #2871      +/-   ##
==========================================
- Coverage   95.46%   95.41%   -0.05%     
==========================================
  Files         260      260              
  Lines       15519    15531      +12     
==========================================
+ Hits        14815    14819       +4     
- Misses        704      712       +8     

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