[PR] Enable BaseSearchCV

[rodrigo-arenas]

This PR implement the missing properties of scikit-learn BaseSearchCV, the parameter return_train_score was added in GASearchCV, the following properties now are calculated as in scikit-learn:

• cvresults
• bestestimator
• bestparams
• bestindex
• scorer_
• nsplits
• refittime

This introduces some changes:

• The GASearchCV.logbook property now has extra information that comes from the scikit-learn cross_validate function
• Internally, now the hof uses the self.best_params_ for the position 0, to be consistent with the scikit-learn API and parameters like self.best_index_.

Tests and documentation is added

[codecov[bot]]

Codecov Report

Merging #55 (c94b103) into master (65611cb) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #55   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        19    +1     
  Lines          669       720   +51     
=========================================
+ Hits           669       720   +51     

Impacted Files	Coverage Δ
sklearn_genetic/genetic_search.py	100.00% <100.00%> (ø)
sklearn_genetic/utils/cv_scores.py	100.00% <100.00%> (ø)

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