Add missing Eigen traits to Jets

Add highest and lowest traits to the Jet implementation.

https://github.com/ceres-solver/ceres-solver/issues/310

Change-Id: I7c68fa8e2baa7742880d3faa21f366352e48aacf

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index 934bc18..ff03f68 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -899,6 +899,9 @@
       Cost = 3
     };
   };
+
+  static inline Real highest() { return Real(std::numeric_limits<T>::max()); }
+  static inline Real lowest() { return Real(-std::numeric_limits<T>::max()); }
 };
 
 #if EIGEN_VERSION_AT_LEAST(3, 3, 0)