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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