Also use the second order term of the series expansion of sin(x/2)/x

Makes for a more precise Hessian matrix when using automatic differentiation.

[[Imported from SVN: r9425]]

dune/gfe/rotation.hh

@@ -143,7 +143,7 @@ class Rotation<T,3> : public Quaternion<T>
 
     /** \brief Computes sin(x/2) / x without getting unstable for small x */
     static T sincHalf(const T& x) {
-        return (x < 1e-4) ? 0.5 - (x*x/48) : std::sin(x/2)/x;
+        return (x < 1e-4) ? 0.5 - (x*x/48)  + (x*x*x*x)/3840 : std::sin(x/2)/x;
     }
 
 public: