API duck-typing for n_neighbors in CNN and deprecate estimator_

[glemaitre]

This PR does:

• deprecate estimator_ in favour of n_neighbors_
• accept any arbitrary knn classifier that has a n_neighbors parameter

[codecov]

Codecov Report

Merging #891 (ba898fb) into master (e802a19) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #891      +/-   ##
==========================================
- Coverage   97.30%   97.29%   -0.01%     
==========================================
  Files          97       97              
  Lines        6337     6352      +15     
  Branches      712      713       +1     
==========================================
+ Hits         6166     6180      +14     
- Misses        102      103       +1     
  Partials       69       69              

Impacted Files	Coverage Δ
...rototype_selection/_condensed_nearest_neighbour.py	100.00% <100.00%> (ø)
...election/tests/test_condensed_nearest_neighbour.py	100.00% <100.00%> (ø)
imblearn/datasets/tests/test_zenodo.py	89.18% <0.00%> (-2.71%)	⬇️

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