Add Solver::Options::IsValid.

This provides a user visible way to validate the Solver::Options
before calling Solve.

Change-Id: Ife84fd33532ab2ccb7ac95abe22735843db51fde

include/ceres/solver.h

@@ -1,5 +1,5 @@
 // Ceres Solver - A fast non-linear least squares minimizer
-// Copyright 2010, 2011, 2012 Google Inc. All rights reserved.
+// Copyright 2014 Google Inc. All rights reserved.
 // http://code.google.com/p/ceres-solver/
 //
 // Redistribution and use in source and binary forms, with or without
@@ -126,6 +126,11 @@ class CERES_EXPORT Solver {
       update_state_every_iteration = false;
     }
 
+    // Returns true if the options struct has a valid
+    // configuration. Returns false otherwise, and fills in *error
+    // with a message describing the problem.
+    bool IsValid(string* error) const;
+
     // Minimizer options ----------------------------------------
 
     // Ceres supports the two major families of optimization strategies -