Add Angle_3(Vector_3, Vector_3)

...by similarity with Angle_2.

Homogeneous_kernel/include/CGAL/Homogeneous/function_objects.h

@@ -86,6 +86,7 @@ namespace HomogeneousKernelFunctors {
   class Angle_3
   {
     typedef typename K::Point_3             Point_3;
+    typedef typename K::Vector_3            Vector_3;
     typedef typename K::Construct_vector_3  Construct_vector_3;
     Construct_vector_3 c;
   public:
@@ -94,6 +95,11 @@ namespace HomogeneousKernelFunctors {
     Angle_3() {}
     Angle_3(const Construct_vector_3& c_) : c(c_) {}
 
+    result_type
+    operator()(const Vector_3& u, const Vector_3& v) const
+    { return enum_cast<Angle>(CGAL_NTS sign(u * v)); }
+    // FIXME: scalar product
+
     result_type
     operator()(const Point_3& p, const Point_3& q, const Point_3& r) const
     { return enum_cast<Angle>(CGAL_NTS sign(c(q,p) * c(q,r))); }