bugfixes: feature selection mutation removal; early stop threshold scoring; misc.

[JensJN]

Addresses several logic bugs, fixing threshold early stopping and mutations in genetic feature selection, as well as miscellaneous other.

Bugfixes:

• Fix inverted threshold logic in early stopping during population selection by adding a not and using >= for correct comparison in evaluate_population_selection_early_stop (base_evolver.py).
• Fix logic flaw in the mutation mask so that features are correctly kept or removed in _mutate_remove (genetic_feature_selection.py).
• Remove ambiguity in selection count in evaluate_population_selection_early_stop (base_evolver.py).

Compatibility:

• Updated the determination of early stop steps to use n_folds for compatibility with cross-validation objects instead of raw integers in objective_function (estimator.py).