Added epsilon() method to NumTraits<ceres::Jet<T, N> >.

Change-Id: Ie849b4ae5c7260976ebf2fb21a00f13d1e6bb647

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index 55caa05..81f96c7 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -649,6 +649,8 @@
     return ceres::Jet<T, N>(1e-12);
   }
 
+  static inline Real epsilon() { return Real(std::numeric_limits<T>::epsilon()); }
+
   enum {
     IsComplex = 0,
     IsInteger = 0,