DropCorrelatedFeatures and SmartCorrelatedSelection consistency changes

[glevv]

Fixes #612

Will also close #619

[codecov[bot]]

Codecov Report

Merging #633 (9ad37aa) into main (ab91403) will increase coverage by 0.12%. The diff coverage is 100.00%.

:exclamation: Current head 9ad37aa differs from pull request most recent head 184525f. Consider uploading reports for the commit 184525f to get more accurate results

@@            Coverage Diff             @@
##             main     #633      +/-   ##
==========================================
+ Coverage   97.90%   98.03%   +0.12%     
==========================================
  Files          98       98              
  Lines        3588     3620      +32     
  Branches      695      707      +12     
==========================================
+ Hits         3513     3549      +36     
+ Misses         28       26       -2     
+ Partials       47       45       -2     

Impacted Files	Coverage Δ
...ature_engine/selection/drop_correlated_features.py	95.45% <100.00%> (+5.45%)	:arrow_up:
...re_engine/selection/smart_correlation_selection.py	100.00% <100.00%> (+2.35%)	:arrow_up:

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[glevv]

Decided to keep only {None, 'nan', 'unique', 'alphabetic'}, since std could be confusing with different scales of features (not standardized) while cv could fix this problem, but will add another if mean is equal (or close) to 0.

[solegalli]

We still need to extend this functionality to the smart correlation selector to merge

[solegalli]

See #648