Move side vertex average normal implementation from edge2 to edge, as

MOOSE has a test using edge3
refs #4257

Author: GiudGiud

include/geom/edge.h

@@ -232,6 +232,8 @@ class Edge : public Elem
 
   virtual bool is_flipped() const override final;
 
+  virtual Point side_vertex_average_normal(const unsigned int s) const override final;
+
   unsigned int center_node_on_side(const unsigned short side) const override final;
 
   ElemType side_type (const unsigned int s) const override final;

include/geom/edge_edge2.h

@@ -146,8 +146,6 @@ class Edge2 : public Edge
    */
   virtual Real volume () const override;
 
-  virtual Point side_vertex_average_normal(const unsigned int s) const override final;
-
 #ifdef LIBMESH_ENABLE_INFINITE_ELEMENTS
 
   /**

src/geom/edge.C

@@ -184,6 +184,14 @@ Edge::is_flipped() const
 }
 
 
+Point
+Edge::side_vertex_average_normal(const unsigned int s) const
+{
+  libmesh_assert_less (s, 2);
+  const auto v = (this->point(0) - this->point(1)).unit();
+  return (s == 0) ? v : -v;
+}
+
 
 bool Edge::on_reference_element(const Point & p,
                                 const Real eps) const

src/geom/edge_edge2.C

@@ -143,14 +143,6 @@ Real Edge2::volume () const
 }
 
 
-Point
-Edge2::side_vertex_average_normal(const unsigned int s) const
-{
-  libmesh_assert_less (s, 2);
-  const auto v = (this->point(0) - this->point(1)).unit();
-  return (s == 0) ? v : -v;
-}
-
 dof_id_type Edge2::key () const
 {
   return this->compute_key(this->node_id(0),