Added missing & symbols to the return types of the matrix cast operators.

Now vector_to_matrix() and array_to_matrix() produce matrix expressions
that return their elements by reference rather than by value.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403024

dlib/matrix/matrix_expressions.h

@@ -1145,12 +1145,12 @@ namespace dlib
         {
         }
 
-        const typename M::value_type operator() (
+        const typename M::value_type& operator() (
             long r, 
             long 
         ) const { return m[r]; }
 
-        const typename M::value_type operator() ( long i ) const 
+        const typename M::value_type& operator() ( long i ) const 
         { return m[i]; }
 
         template <typename U, long iNR, long iNC, typename MM, typename L>
@@ -1228,12 +1228,12 @@ namespace dlib
         {
         }
 
-        const typename M::type operator() (
+        const typename M::type& operator() (
             long r, 
             long 
         ) const { return m[r]; }
 
-        const typename M::type operator() ( long i ) const 
+        const typename M::type& operator() ( long i ) const 
         { return m[i]; }
 
         template <typename U, long iNR, long iNC, typename MM, typename L>
@@ -1311,7 +1311,7 @@ namespace dlib
         {
         }
 
-        const typename M::type operator() (
+        const typename M::type& operator() (
             long r, 
             long c
         ) const { return m[r][c]; }