Add const accessor for functor wrapped by auto/numeric-diff objects

- The intended use-case for these accessors is in client code tests to
  support verification of the configuration with which cost functions
  were constructed.

Change-Id: Ib77afa6409804ba7f724138f579e0c51b154f5ad

include/ceres/autodiff_first_order_function.h

@@ -142,6 +142,8 @@ class AutoDiffFirstOrderFunction : public FirstOrderFunction {
 
   int NumParameters() const override { return kNumParameters; }
 
+  const FirstOrderFunctor & functor() const { return *functor_; }
+
  private:
   std::unique_ptr<FirstOrderFunctor> functor_;
 };