Use override & final instead of just using virtual.

This is safer than using virtual and this lead to a minor
bug fixes.

Change-Id: Id69cb1cc569bf6bf245f22f029c7871b6c712568

internal/ceres/dynamic_numeric_diff_cost_function_test.cc

@@ -310,7 +310,7 @@ class MyThreeParameterCostFunctor {
 
 class ThreeParameterCostFunctorTest : public ::testing::Test {
  protected:
-  virtual void SetUp() {
+  void SetUp() final {
     // Prepare the parameters.
     x_.resize(1);
     x_[0] = 0.0;