Preprocessing - improve additive smoothing in target encoder

[sandervh14]

Preprocessing: improve additive smoothing in target encoder

Task Description

Additive smoothing in the target encoder prevents possible overfitting, but is disabled by default, with the default weight=0 parameter setting. Improve the approach, see comments https://github.com/PythonPredictions/cobra/issues/67#issuecomment-894122194 and https://github.com/PythonPredictions/cobra/issues/67#issuecomment-894077255.