Add ttest

[FranziskaWinterstein]

Description

This new preprocessor function calculates a t-test and adds the resulted p-value as an Ancillary Variable to the model cubes. This p-value can later be used for hatching non-significant values when plotting the bias of model and reference. It is planned to add such in monitor/multi_datasets.py.

Link to documentation:

Closes #3132 3132

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[FranziskaWinterstein]

Currently the processing fails with

  File "/work/bd1132/b309109/conda_envs/envs/esmvaltool/lib/python3.13/site-packages/numpy/_core/fromnumeric.py", line 86, in _wrapreduction
    return ufunc.reduce(obj, axis, dtype, out, **passkwargs)
      ^^^^^^^^^^^^^^^^^
numpy.exceptions.AxisError: <exception str() failed>

see also attached main_log_debug.txt

[codecov]

Codecov Report

❌ Patch coverage is 26.08696% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.03%. Comparing base (d7e7586) to head (2beb91e).

Files with missing lines	Patch %	Lines
esmvalcore/preprocessor/_compare_with_refs.py	24.44%	34 Missing ⚠️

❌ Your patch check has failed because the patch coverage (26.08%) 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    #3133      +/-   ##
==========================================
- Coverage   96.23%   96.03%   -0.21%     
==========================================
  Files         273      273              
  Lines       16132    16174      +42     
==========================================
+ Hits        15525    15532       +7     
- Misses        607      642      +35     

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