Add toString method to WeakClassifier
Move variable d to constant TRUE_POSITIVE_DECREASE_TOLERANCE
Overhaul of error computations

(Technically not a refactorization, since this changes the functionality)

Changed the way errors are computed to that suggested in the lab manual.
This also gives an explicit way to select the parity.
selectAndTrainWeakClassifier has been completely rewritten, and some
helper methods have been changed.
Add constant for training data file; add some output to Training#main