Fix Jet/Eigen compatibility for Eigen > 3.3

Honour the digits10 NumTraits member introduced in Eigen 3.3.

Change-Id: If20183332beef8335b7758cdb58859a43fc59970

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index 93cdd72..27c39d3 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -850,6 +850,8 @@
     return Real(std::numeric_limits<T>::epsilon());
   }
 
+  static inline int digits10() { return NumTraits<T>::digits10(); }
+
   enum {
     IsComplex = 0,
     IsInteger = 0,