full script 3 review

This is only script 3 in-depth review (done previously by Tarun, not me).

Includes:

• layout updates for using TOC
• multiple minor typos, content and code bugs fixes (e.g. correct for no shuffling on cross_val_score by default)
• some code simplifications (e.g. no numpy.random)
• extension for shuffling and stratification on splits, both train/test and CV (includes use of explicit cross-validator)