Fix bug in method lowerTriangularSolve

This makes triangularsolvetest.cc pass.

dune/matrix-vector/triangularsolve.hh

@@ -37,7 +37,7 @@ namespace MatrixVector {
           // Note: We could drop the check for ignore nodes here bcs. b[j] will
           // be ignored anyway due to the check above.
           if (ignore == nullptr or (*ignore)[j].none())
-            b[j] -= x[i] * *cIt;
+            cIt->mmv(x[i], b[j]);
         }
       }
     } else {