Replace addToDiagonal by += in IPDG assembler

The code erroneously used the addToDiagonal method from the
dune-matrix-vector module to add two objects of type
FieldMatrix<T,1,1>.  That worked nicely, until recently
the addToDiagonal input data checks were tightened:
Now the matrix type and the value to be added to the diagonal
can only be of the same type if both are number types.
As FM<,1,1> is not a number type the test in
dunefunctionsipdgassemblertest.cc started not to compile
anymore.

Fix this by replacing addToDiagonal by operator+=.

dune/fufem/assemblers/localassemblers/interiorpenaltydgassembler.hh

@@ -216,18 +216,10 @@ class InteriorPenaltyDGAssembler : public LocalAssembler<GridType, TrialLocalFE,
           // Depending on which elements i and j belong to, perform different operations.
           // In particular, i (resp. j) < insizeSize corresponds to functions from the inner element
           // while the others correspond to the outer element
-          if (i < insideSize){
-            if(j< insideSize)
-              Dune::MatrixVector::addToDiagonal(localMatrix[i][j], mc[0][0][i][j]);
+          if (i < insideSize)
+            localMatrix[i][j] += (j< insideSize) ?  mc[0][0][i][j] : mc[0][1][i][j-insideSize];
           else
-              Dune::MatrixVector::addToDiagonal(localMatrix[i][j], mc[0][1][i][j-insideSize]);
-          }
-          else {
-            if(j< insideSize)
-              Dune::MatrixVector::addToDiagonal(localMatrix[i][j], mc[1][0][i-insideSize][j]);
-            else
-              Dune::MatrixVector::addToDiagonal(localMatrix[i][j], mc[1][1][i-insideSize][j-insideSize]);
-          }
+            localMatrix[i][j] += (j< insideSize) ? mc[1][0][i-insideSize][j] : mc[1][1][i-insideSize][j-insideSize];
         }
       }
     }