Modified the probabilistic() trainer adapter (and the train_probabilistic_decision_function() routine)
so that they work with objects which have an interface compatible with std::vector rather than strictly
just std::vector objects.  For example, random_subset_selector objects can be used (as was the case in
some previous versions of dlib).

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404211