Improvements in decision trees - classification part 2 script

• remove pydot in favour of sklearn.tree.plot_tree
• improve exercises: no useless min_samples_leaf; add info on tree size measures
• improve multi-tree plots (horizontal scrolling)
• explicit info on RF and boosting ensemble sizes
• better stacking description: motivation and performance expectation

changed milestone to %Workshop 3rd edition (2020-2021)

created merge request !35 (merged) to address this issue

mentioned in merge request !35 (merged)

closed with merge request !35 (merged)

mentioned in commit 27626943