Feature extraction speedup, bugfixes and model code.

This release provides:

• a significant speedup and memory reduction of the feature extraction phase,
• bugfixes in the feature extraction pipeline,
• the code of the original model architecture (tensorflow keras),
• alignment of the SherlockModel class with the scikit-learn API (i.e. w/ fit, predict, predict_proba methods),
• improved notebooks demonstrating 1) full reproduction of the feature extraction and model training/evaluation pipelines, 2) out-of-the-box usage of the Sherlock model for a given table, 3) how performance can be improved with additional classifiers.

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Contributions by:
@lowecg
@madelonhulsebos