implement train_test_split keyword

[aaronspring]

Description

fix partly #648

• implement train_test_split
• check similar sizes
• fair
• fair-sliding
• fair-all
• works with alignment?
• check again same_verifs
• document, checknotebook, similar plots to Risbey et al. 2021

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

• pytest

Checklist (while developing)

• I have added docstrings to all new functions.
• I have commented my code, particularly in hard-to-understand areas
• Tests added for pytest, if necessary.
• I have updated the sphinx documentation, if necessary.
• Any new functions are added to the API. (See contribution guide)
• CHANGELOG is updated with reference to this PR.

References

http://www.nature.com/articles/s41467-021-23771-z

Demo

https://gist.github.com/aaronspring/bc001bd11d742cfc65d55f8b09d9c51a

RMSE for all NMME models:

[codecov]

Codecov Report

Merging #655 (ba69d5c) into main (2e27c9d) will decrease coverage by 0.07%.
The diff coverage is 97.03%.

@@            Coverage Diff             @@
##             main     #655      +/-   ##
==========================================
- Coverage   93.48%   93.41%   -0.08%     
==========================================
  Files          56       56              
  Lines        5559     5646      +87     
==========================================
+ Hits         5197     5274      +77     
- Misses        362      372      +10     

Impacted Files	Coverage Δ
climpred/metrics.py	90.96% <ø> (ø)
climpred/tutorial.py	53.84% <ø> (ø)
climpred/checks.py	94.57% <75.00%> (-1.30%)	⬇️
climpred/classes.py	94.01% <95.23%> (+0.34%)	⬆️
climpred/bias_removal.py	93.62% <98.27%> (-3.04%)	⬇️
climpred/constants.py	100.00% <100.00%> (ø)
climpred/tests/test_bias_removal.py	100.00% <100.00%> (ø)
climpred/tests/test_options.py	95.00% <0.00%> (-5.00%)	⬇️

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