Update Target Encoding tutorials with new TE API

[ledell]

New Target Encoding API in 3.32 requires a rewrite of these tutorials:

• https://github.com/h2oai/h2o-tutorials/blob/master/best-practices/feature-engineering/feature_engineering.ipynb
• https://github.com/h2oai/h2o-tutorials/tree/master/best-practices/categorical-predictors