Add a typedef to expose the scalar type used in a Jet.

Change-Id: I4232d76619f4f9c48a0c716150b8c5ffc8b74462

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index 0d5c780..7eb199a 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -170,6 +170,7 @@
 template <typename T, int N>
 struct Jet {
   enum { DIMENSION = N };
+  typedef T Scalar;
 
   // Default-construct "a" because otherwise this can lead to false errors about
   // uninitialized uses when other classes relying on default constructed T