Remove conversion operator

A FieldMatrix can still have a DiagonalMatrix assigned to it through the
new DenseMatrixAssigner specialisation. The restriction from general
DenseMatrix implementations to the FieldMatrix class is removed in the
process.

dune/common/identitymatrix.hh

 #ifndef DUNE_COMMON_IDENTITYMATRIX_HH
 #define DUNE_COMMON_IDENTITYMATRIX_HH
 
+#include <dune/common/boundschecking.hh>
 #include <dune/common/fmatrix.hh>
 #include <dune/common/ftraits.hh>
 #include <dune/common/math.hh>
@@ -146,17 +147,18 @@ namespace Dune
     {
       return FieldTraits< field_type >::real_type( 1 );
     }
+  };
 
-    /** \brief cast to FieldMatrix */
-    operator FieldMatrix< field_type, N, N > () const
-    {
-      FieldMatrix< field_type, N, N > fieldMatrix( 0 );
-      for( int i = 0; i < N; ++i )
-        fieldMatrix[ i ][ i ] = field_type( 1 );
-      return fieldMatrix;
+  template <class DenseMatrix, class field, int N>
+  struct DenseMatrixAssigner<DenseMatrix, IdentityMatrix<field, N>> {
+    static void apply(DenseMatrix &denseMatrix, IdentityMatrix<field, N> const &rhs) {
+      DUNE_ASSERT_BOUNDS(denseMatrix.M() == N);
+      DUNE_ASSERT_BOUNDS(denseMatrix.N() == N);
+      denseMatrix = field(0);
+      for (int i = 0; i < N; ++i)
+        denseMatrix[i][i] = field(1);
     }
   };
-
 } // namespace Dune
 
 #endif // #ifndef DUNE_COMMON_IDENTITYMATRIX_HH