Implement the side vertex average normal for edge3

see #4257
refs idaholab/moose#31554

Author: GiudGiud

include/geom/edge_edge3.h

@@ -166,6 +166,8 @@ class Edge3 : public Edge
    */
   virtual Real volume () const override;
 
+  virtual Point side_vertex_average_normal(const unsigned int s) const override final;
+
   /**
    * \returns A bounding box (not necessarily the minimal bounding box)
    * containing the edge.

src/geom/edge_edge3.C

@@ -265,6 +265,14 @@ Real Edge3::volume () const
 }
 
 
+Point
+Edge3::side_vertex_average_normal(const unsigned int s) const
+{
+  libmesh_assert_less (s, 2);
+  libmesh_assert_equal_to(this->mapping_type(), LAGRANGE_MAP);
+  return (s == 0) ? (this->point(0) - this->point(2)).unit() : (this->point(1) - this->point(2)).unit();
+}
+
 
 BoundingBox Edge3::loose_bounding_box () const
 {