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.

This works well with masked/non-masked and lazy/non-lazy data (also tested with distributed schedulers).

Link to documentation:

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

Closes #3132

Test recipe:

# ESMValTool
---
documentation:
  description: Test
  authors:
    - schlund_manuel
  title: Test.

datasets:
  - {dataset: BCC-ESM1, project: CMIP6, exp: historical, ensemble: r1i1p1f1, grid: gn, timerange: 2000/2005}
  - {dataset: bcc-csm1-1, version: v1, project: CMIP5, exp: historical, ensemble: r1i1p1, reference_for_t_test: true, timerange: 2000/2005}

preprocessors:
  t-test:
    regrid:
      target_grid: 10x10
      scheme: linear
    mask_landsea:
      mask_out: sea
    t_test:
      coords: [time]
      # nan_policy: omit  # omit: masked values are ignored in t-test calculation
      # equal_var: false  # false: perform Welch's t-test

diagnostics:

  t-test:
    variables:
      tas:
        mip: Amon
        timerange: 1850/2005
        preprocessor: t-test
    scripts:
      null

---

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---

[CLAassistant]

All committers have signed the CLA.

[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 96.31%. Comparing base (08a8dbb) to head (cff6911).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3133      +/-   ##
==========================================
+ Coverage   96.30%   96.31%   +0.01%     
==========================================
  Files         280      280              
  Lines       16252    16310      +58     
==========================================
+ Hits        15651    15709      +58     
  Misses        601      601              

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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

[schlunma]

This is ready for review now.