Adjust test to interface changes

After 452b62b9b57362bbb0cf16a5dd48804132d74615

src/test-gradient-method.cc

@@ -35,7 +35,7 @@ void testIdentity() {
   b[0] = 1;
   b[1] = 2;
 
-  Dune::Identity f;
+  Dune::LinearFunction f(1);
   Functional J(A, b, Dune::MyNonlinearity<dim>(f));
 
   Functional::SmallVector start = b;