add monthly precipitation data for tests

[sarasita]

The files were generated using cdo remapbil,r20x20 /net/atmos/data/cmip6-ng/pr/mon/g025/pr_mon_IPSL-CM6A-LR_historical_r1i1p1f1_g025.nc /home/scsarah/mesmer/tests/test-data/calibrate-coarse-grid/cmip6-ng/pr/mon/g025/pr_mon_IPSL-CM6A-LR_historical_r1i1p1f1_g025.nc

• Closes #xxx
• Tests added
• Fully documented, including CHANGELOG.rst

[mathause]

Note that the data is also in #837

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.49%. Comparing base (24c00f1) to head (34922c7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #838   +/-   ##
=======================================
  Coverage   99.49%   99.49%           
=======================================
  Files          31       31           
  Lines        1992     1992           
=======================================
  Hits         1982     1982           
  Misses         10       10           

Flag	Coverage Δ
unittests	99.49% <ø> (ø)

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