text: Title: replace concat in encoders by y_train.groupby(X_train["variable"]).mean().sort_values()
  Body: Applies to the following encoders:
  
  - OrdinalEncoder
  - MeanEncoder
  - PRatioEncoder
  - WoEEncoder
  
  Currently we concatenate the target to the predictors (X_train) to determine the mappings. This is not necessary. We can determined them automatically with syntax like this:
  
  y_train.groupby(X_train["A7"]).mean().sort_values()
  
  therefore removing one unnecessary step in the calculation, the pd.concat step