Evaluation callback API

This adds a callback mechanism to for users to get notified just
before jacobian and residual evaluations. This will enable
aggressive caching and sharing of compute between cost functions.

Change-Id: I67993726920218edf71ab9ae70c34c204756c71a

internal/ceres/evaluator.h

diff --git a/internal/ceres/evaluator.h b/internal/ceres/evaluator.h
index cbb7aa2..532f437 100644
--- a/internal/ceres/evaluator.h
+++ b/internal/ceres/evaluator.h
@@ -44,6 +44,7 @@
 namespace ceres {
 
 struct CRSMatrix;
+class EvaluationCallback;
 
 namespace internal {
 
@@ -64,13 +65,15 @@
           num_eliminate_blocks(-1),
           linear_solver_type(DENSE_QR),
           dynamic_sparsity(false),
-          context(NULL) {}
+          context(NULL),
+          evaluation_callback(NULL) {}
 
     int num_threads;
     int num_eliminate_blocks;
     LinearSolverType linear_solver_type;
     bool dynamic_sparsity;
     ContextImpl* context;
+    EvaluationCallback* evaluation_callback;
   };
 
   static Evaluator* Create(const Options& options,
@@ -98,12 +101,16 @@
   // Options struct to control Evaluator::Evaluate;
   struct EvaluateOptions {
     EvaluateOptions()
-        : apply_loss_function(true) {
+        : apply_loss_function(true),
+          new_evaluation_point(true) {
     }
 
     // If false, the loss function correction is not applied to the
     // residual blocks.
     bool apply_loss_function;
+
+    // If false, this evaluation point is the same as the last one.
+    bool new_evaluation_point;
   };
 
   // Evaluate the cost function for the given state. Returns the cost,