Added a value_type to the random_subset_selector so that it is more compatible

with the std::vector interface.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404210

dlib/statistics/random_subset_selector.h

@@ -35,6 +35,7 @@ namespace dlib
         !*/
     public:
         typedef T type;
+        typedef T value_type;
         typedef default_memory_manager mem_manager_type;
         typedef Rand_type rand_type;
 

dlib/statistics/random_subset_selector_abstract.h

@@ -71,6 +71,7 @@ namespace dlib
         !*/
     public:
         typedef T type;
+        typedef T value_type;
         typedef default_memory_manager mem_manager_type;
         typedef Rand_type rand_type;