DOC add test for numpydoc validation and documented param/attributes

[glemaitre]

Make sure that we validate both the functions and classes with numpydoc

[codecov]

Codecov Report

Merging #869 (34837ce) into master (56eefdf) will decrease coverage by 0.27%.
The diff coverage is 86.57%.

@@            Coverage Diff             @@
##           master     #869      +/-   ##
==========================================
- Coverage   98.84%   98.56%   -0.28%     
==========================================
  Files          93       94       +1     
  Lines        6064     6209     +145     
  Branches      508      543      +35     
==========================================
+ Hits         5994     6120     +126     
- Misses         69       77       +8     
- Partials        1       12      +11     

Impacted Files	Coverage Δ
imblearn/combine/_smote_enn.py	100.00% <ø> (ø)
imblearn/combine/_smote_tomek.py	100.00% <ø> (ø)
imblearn/datasets/_imbalance.py	88.23% <ø> (ø)
imblearn/datasets/_zenodo.py	96.77% <ø> (ø)
imblearn/ensemble/_bagging.py	98.07% <ø> (ø)
imblearn/ensemble/_easy_ensemble.py	100.00% <ø> (ø)
imblearn/ensemble/_weight_boosting.py	97.75% <ø> (ø)
imblearn/exceptions.py	100.00% <ø> (ø)
imblearn/metrics/_classification.py	96.34% <ø> (ø)
imblearn/metrics/pairwise.py	100.00% <ø> (ø)
... and 22 more

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