Add an ExecutionSummary object that the Evaluator and LinearSolver can use to
report execution statistics of all kinds.

Currently a single map which maps arbitrary strings to doubles is supported,
which allows for precise timing information to be communicated.

Change-Id: Ibd930aca5c9e6cae89bcfeffe9b13e2887644881

internal/ceres/linear_solver.h

diff --git a/internal/ceres/linear_solver.h b/internal/ceres/linear_solver.h
index ee7fce2..6e03f60 100644
--- a/internal/ceres/linear_solver.h
+++ b/internal/ceres/linear_solver.h
@@ -42,6 +42,7 @@
 #include "ceres/casts.h"
 #include "ceres/compressed_row_sparse_matrix.h"
 #include "ceres/dense_sparse_matrix.h"
+#include "ceres/execution_summary.h"
 #include "ceres/triplet_sparse_matrix.h"
 #include "ceres/types.h"
 
@@ -255,6 +256,8 @@
                         const PerSolveOptions& per_solve_options,
                         double* x) = 0;
 
+  virtual const ExecutionSummary ExecutionSummary() const { return ExecutionSummary(); }
+
   // Factory
   static LinearSolver* Create(const Options& options);
 };