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/program_evaluator.h

@@ -85,6 +85,7 @@
 #include <map>
 #include <string>
 #include <vector>
+#include "ceres/evaluation_callback.h"
 #include "ceres/execution_summary.h"
 #include "ceres/internal/eigen.h"
 #include "ceres/internal/scoped_ptr.h"
@@ -156,6 +157,14 @@ class ProgramEvaluator : public Evaluator {
       return false;
     }
 
+    // Notify the user about a new evaluation point if they are interested.
+    if (options_.evaluation_callback != NULL) {
+      program_->CopyParameterBlockStateToUserState();
+      options_.evaluation_callback->PrepareForEvaluation(
+          /*jacobians=*/(gradient != NULL || jacobian != NULL),
+          evaluate_options.new_evaluation_point);
+    }
+
     if (residuals != NULL) {
       VectorRef(residuals, program_->NumResiduals()).setZero();
     }