Add a custom date2num function to deal with changes in cftime

[zklaus]

Description

Closes #1372

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[codecov]

Codecov Report

Merging #1373 (68dc5bf) into main (639e5ab) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1373      +/-   ##
==========================================
+ Coverage   88.37%   88.39%   +0.01%     
==========================================
  Files         195      195              
  Lines        9943     9958      +15     
==========================================
+ Hits         8787     8802      +15     
  Misses       1156     1156              

Impacted Files	Coverage Δ
esmvalcore/cmor/_fixes/cmip5/gfdl_cm2p1.py	83.33% <100.00%> (+0.40%)	⬆️
esmvalcore/cmor/_fixes/cmip6/fgoals_f3_l.py	86.95% <100.00%> (+0.59%)	⬆️
esmvalcore/cmor/_fixes/icon/icon.py	100.00% <100.00%> (ø)
esmvalcore/cmor/_fixes/native6/era5.py	90.75% <100.00%> (+0.03%)	⬆️
esmvalcore/cmor/_fixes/native6/mswep.py	77.46% <100.00%> (+0.32%)	⬆️
esmvalcore/cmor/check.py	97.72% <100.00%> (+<0.01%)	⬆️
esmvalcore/iris_helpers.py	100.00% <100.00%> (ø)
esmvalcore/preprocessor/_multimodel.py	95.00% <100.00%> (+0.03%)	⬆️
esmvalcore/preprocessor/_time.py	94.75% <100.00%> (+0.03%)	⬆️

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[valeriupredoi]

cheers @zklaus - tests/unit/preprocessor/_multimodel/test_multimodel.py is still using num2date (not date2num) - do you think we should use the num2date from iris too? It's the reciprocal function so it's a sensible idea methinks

[zklaus]

@valeriupredoi, it's not a bad idea per se, but since this is a hotfix and num2date doesn't suffer from the problem, let's defer that. Feel free to open an issue about it.

[valeriupredoi]

happy to see cftime go at least just for date2num - we still have two instances of importing and using the module in cmor/_fixes/cmip6/fgoals_f3_l.py and cmor/_fixes/cmip5/gfdl_cm2p1.py and one test in tests/unit/preprocessor/_multimodel/test_multimodel.py - I think we can make sure those get replaced too (maybe, not sure about cftime.DatetimeJulian or cftime.DatetimeNoLeap), but anyway that'd be done in a separate PR. Klaus, could you add a docstring to the new helper function (bit of say why we use that), all good by me other than that 🍺

[valeriupredoi]

@zklaus yer, will do, given how shifty-mcshiftface cftime is I' be quite happy to see if we can wholly get rid of it 👍

[zklaus]

I'll just add a test and then we're good to go.

[valeriupredoi]

I'll just add a test and then we're good to go.

yeah could do, but that's tested via iris so maybe even no need