Made the matrix constructor that uses C arrays explicit and added a COMPILE_TIME_ASSERT

to the scalar*matrix multiplication code so that a user gets a more straight forward
error when they try to multiply a scalar times a matrix that contains matrices.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403365

dlib/matrix/matrix.h

@@ -776,6 +776,8 @@ namespace dlib
         const static long cost = matrix_traits<matrix_mul_scal_exp>::cost;
         typedef typename matrix_traits<matrix_mul_scal_exp>::layout_type layout_type;
 
+        // You aren't allowed to multiply a matrix of matrices by a scalar.   
+        COMPILE_TIME_ASSERT(is_matrix<type>::value == false);
 
         // This constructor exists simply for the purpose of causing a compile time error if
         // someone tries to create an instance of this object with the wrong kind of objects.
@@ -1103,7 +1105,7 @@ namespace dlib
         }
 
         template <typename U, size_t len>
-        matrix (
+        explicit matrix (
             U (&array)[len]
         ) 
         {

dlib/matrix/matrix_abstract.h

@@ -432,7 +432,7 @@ namespace dlib
         !*/
 
         template <typename U, size_t len>
-        matrix (
+        explicit matrix (
             U (&array)[len]
         );
         /*!