Add GradientProblemSolver::Options::parameter_tolerance.

Support for parameter tolerance was added to the line search
minimizer was added a while ago, and calling Solve on a
non-linear least squares problem supported it but for some reason
the GradientProblemSolver::Options struct was missing this
option even though the documentation suggested that it was present!

Thanks to Noah Snavely for reporting this bug.

Change-Id: I57cf4ab396bc822c19fa298529e113b89664a349

include/ceres/gradient_problem_solver.h

diff --git a/include/ceres/gradient_problem_solver.h b/include/ceres/gradient_problem_solver.h
index a7d0121..546dc34 100644
--- a/include/ceres/gradient_problem_solver.h
+++ b/include/ceres/gradient_problem_solver.h
@@ -74,6 +74,7 @@
       max_solver_time_in_seconds = 1e9;
       function_tolerance = 1e-6;
       gradient_tolerance = 1e-10;
+      parameter_tolerance = 1e-8;
       logging_type = PER_MINIMIZER_ITERATION;
       minimizer_progress_to_stdout = false;
     }
@@ -236,6 +237,12 @@
     // This value should typically be 1e-4 * function_tolerance.
     double gradient_tolerance;
 
+    // Minimizer terminates when
+    //
+    //   |step|_2 <= parameter_tolerance * ( |x|_2 +  parameter_tolerance)
+    //
+    double parameter_tolerance;
+
     // Logging options ---------------------------------------------------------
 
     LoggingType logging_type;

internal/ceres/gradient_problem_solver.cc

diff --git a/internal/ceres/gradient_problem_solver.cc b/internal/ceres/gradient_problem_solver.cc
index 8709f8f..018401d 100644
--- a/internal/ceres/gradient_problem_solver.cc
+++ b/internal/ceres/gradient_problem_solver.cc
@@ -72,6 +72,7 @@
   COPY_OPTION(max_line_search_step_expansion);
   COPY_OPTION(max_num_iterations);
   COPY_OPTION(max_solver_time_in_seconds);
+  COPY_OPTION(parameter_tolerance);
   COPY_OPTION(function_tolerance);
   COPY_OPTION(gradient_tolerance);
   COPY_OPTION(logging_type);