Pretty tree: prune branches with identical prediction

[mathias-brandewinder]

The current Pretty Tree sometimes keeps searching and rendering branches, although every single Leaf (for instance) has the same default outcome [see for instance the Tree obtained on Titanic with Sex, Class]. Having a "compact" Tree pruning the "uninformative" branches (i.e. branches where every single Leaf has the same prediction) would make for a more readable output.