Added doxygen comment and assertion

[[Imported from SVN: r1401]]

fem/operator/massmatrix.hh

@@ -40,7 +40,7 @@ namespace Dune {
       return this->matrix_;
     }
 
-    //! ???
+    //! Constructor
     MassMatrixFEOp( const typename DiscFunctionType::FunctionSpace &f, OpMode opMode ) : //= ON_THE_FLY ) :
                                                                                         FiniteElementOperator<DiscFunctionType,
                                                                                             SparseRowMatrix<double>,
@@ -95,6 +95,8 @@ namespace Dune {
       const double vol = entity.geometry().integration_element(tmp);
 
       static RangeType v[4];
+      // Check magic constant. Otherwise program will fail in loop below
+      assert(matSize <= 4);
 
       for(i=0; i<matSize; i++)
         for (j=0; j<=i; j++ )