with any kind of trainer object rather than only ones which produce
   dlib::decision_function objects.  I also made it work with trainers that only
   take std::vectors.
 - train_probabilistic_decision_function() no longer accepts column vectors of
   samples and labels.  Now it only accepts std::vectors of samples and labels.
 - Added a new generic_probabilistic_decision_function object which is returned by
   the train_probabilistic_decision_function().  The old probabilistic_decision_function
   still exists and can be constructed from a generic_probabilistic_decision_function.
   This way backwards compatibility is maintained with older code so long as that
   code used std::vectors to hold samples and labels.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404072