Pipeline smote fix

[elemets]

Contains all previous fixes relating to:

• Catboost support (early stopping, and support involving resetting estimators).
• Pipeline steps now support hyperparameter tuning of the resamplers (SMOTE, ADASYN, etc.).
• Removed older implementations of impute and scaling and moved onto supporting only custom pipeline steps.
• Fixed bugs in stratification with regards to length mismatch of dependent variable when using column names to stratify.
• Cleaned a removed multiple lines of unused code and unused initialisation parameters.

TODO:

• Remove code relating to dropping stratified columns.
• Convert selectKBest into a more generalised parameter relating to using some form of feature elimination.

[lshpaner]

Nothing substantial beyond removing conflicting code re: future deprecation of drop_strat_feat