GradientProblem & related classes use std::unique_ptr

Previously these classes in analogy with ceres::Problem's interface
had interfaces to allow bare pointers as well as unique_ptrs. This
CL changes the API to always use unique_ptr, this is less error prone
and makes the default ownership semantics clearer.

Change-Id: I7577a90761f341c7e009c248c820f0fec2e6f32d

examples/rosenbrock_numeric_diff.cc

@@ -47,12 +47,12 @@ struct Rosenbrock {
     return true;
   }
 
-  static ceres::FirstOrderFunction* Create() {
+  static std::unique_ptr<ceres::FirstOrderFunction> Create() {
     constexpr int kNumParameters = 2;
-    return new ceres::NumericDiffFirstOrderFunction<Rosenbrock,
-                                                    ceres::CENTRAL,
-                                                    kNumParameters>(
-        new Rosenbrock);
+    return std::make_unique<
+        ceres::NumericDiffFirstOrderFunction<Rosenbrock,
+                                             ceres::CENTRAL,
+                                             kNumParameters>>();
   }
 };