Made the probabilistic trainer adapter not hold onto the input trainer

by reference since this causes problems if the probabilistic adapter
is saved for later use.  Now it will simply hold the trainer by value.

dlib/svm/svm.h

@@ -818,7 +818,7 @@ namespace dlib
     {
         typedef probabilistic_function<typename trainer_type::trained_function_type> trained_function_type;
 
-        const trainer_type& trainer;
+        const trainer_type trainer;
         const long folds;
 
         trainer_adapter_probabilistic (