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_cost_function.h

@@ -215,6 +215,8 @@ class AutoDiffCostFunction : public SizedCostFunction<kNumResiduals, Ns...> {
         jacobians);
   };
 
+  const CostFunctor & functor() const { return *functor_; }
+
  private:
   std::unique_ptr<CostFunctor> functor_;
   Ownership ownership_;