Thanks, @jmnybl , for explaining the negative tau situation. Confident now to silently refuse to update if negative tau occurs.
Save the model after every iteration by default.

Can be switched off with --no_save_by_iter
Optimizing feature generation.
(cherry picked from commit dc1c651f8b6aa284da88b53ad650610707cf01a8)
Merge branch 'master' of github.com:jmnybl/Turku-Dependency-Parser
Shared feature generation.
Conflicts solved.