Fixed a bug in the kernel_matrix() function. It didn't compile when used with

sparse samples which were of type std::vector<std::pair<> >.  Moreover, some of
the trainers have a dependency on kernel_matrix() so this fix makes those trainers
also work with this kind of sparse sample.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404164

dlib/svm/kernel_matrix.h

@@ -30,14 +30,18 @@ namespace dlib
             return m[i];
         }
 
+        // Only use this function if T isn't a std::pair because in that case the entire vector is
+        // probably itself a sparse sample.
         template <typename kernel_type, typename T, typename alloc>
-        inline const T& access ( const std::vector<T,alloc>& m, long i)
+        inline typename disable_if<is_pair<T>,const T&>::type access ( const std::vector<T,alloc>& m, long i)
         {
             return m[i];
         }
 
+        // Only use this function if T isn't a std::pair because in that case the entire vector is
+        // probably a sparse sample.
         template <typename kernel_type, typename T, typename alloc>
-        inline const T& access ( const std_vector_c<T,alloc>& m, long i)
+        inline typename disable_if<is_pair<T>,const T&>::type access ( const std_vector_c<T,alloc>& m, long i)
         {
             return m[i];
         }