Added a value_type typedef to matrix_exp so it's easier to write templates

which operate on STL containers and matrix objects.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404166

dlib/matrix/matrix_exp.h

@@ -67,6 +67,7 @@ namespace dlib
 
     public:
         typedef typename matrix_traits<EXP>::type type;
+        typedef type value_type;
         typedef typename matrix_traits<EXP>::const_ret_type const_ret_type;
         typedef typename matrix_traits<EXP>::mem_manager_type mem_manager_type;
         typedef typename matrix_traits<EXP>::layout_type layout_type;

dlib/matrix/matrix_exp_abstract.h

@@ -48,6 +48,7 @@ namespace dlib
 
     public:
         typedef typename EXP::type type;
+        typedef type value_type; // Redefined for compatibility with the STL
         typedef typename EXP::const_ret_type const_ret_type;
         typedef typename EXP::mem_manager_type mem_manager_type;
         typedef typename EXP::layout_type layout_type;