Add preprocessor t_test

[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:

• https://esmvaltool--3133.org.readthedocs.build/projects/ESMValCore/en/3133/api/esmvalcore.preprocessor.html#esmvalcore.preprocessor.t_test

Closes #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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.24%. Comparing base (87ba05a) to head (04006d7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3133      +/-   ##
==========================================
+ Coverage   96.22%   96.24%   +0.01%     
==========================================
  Files         273      273              
  Lines       16150    16202      +52     
==========================================
+ Hits        15541    15593      +52     
  Misses        609      609              

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[schlunma]

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

This should be fixed now.

I also updated the code so it properly works with lazy/non-lazy and masked/non-masked data, and added corresponding tests.

Would be great if you can check if it still works as you would expect!

Note that I renamed the preprocessor to t_test to avoid 100s of errors from codespell. This is probably also more readable.

Still missing:

• Docs: https://github.com/ESMValGroup/ESMValCore/blob/main/doc/recipe/preprocessor.rst
• Recipe-level check so errors can be caught early on similar to the bias check