Added a precondition for Arrangement_on_surface_2::merge_edge(e1, e2) (#8742)

Added a precondition for merge_edge(e1, e2) that requires that two
halfedges have the same direction.

## Summary of Changes

The title says it all

## Release Management

* Affected package(s): Arrangement_on_surface_2
* Issue(s) solved (if any): fix #8659
* Feature/Small Feature (if any): 
* Link to compiled documentation:
* License and copyright ownership: TAU

Author: sloriot

Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arrangement_on_surface_2.h

@@ -973,6 +973,7 @@ class Arrangement_on_surface_2 {
    * u_1\f$ to \f$ u_2\f$.
    * \pre `e1` and `e2` share a common end-vertex, such that the two other
    * end-vertices of the two edges are associated with `c`'s endpoints.
+   * \pre `e1` and `e2` have the same direction.
    */
   Halfedge_handle merge_edge(Halfedge_handle e1,
                              Halfedge_handle e2,

Arrangement_on_surface_2/include/CGAL/Arr_counting_traits_2.h

@@ -411,7 +411,7 @@ class Arr_counting_traits_2 : public BaseTraits {
       m_object(base.construct_opposite_2_object()), m_counter(counter) {}
 
     /*! operates */
-    X_monotone_curve_2 operator()(const X_monotone_curve_2& xc)
+    X_monotone_curve_2 operator()(const X_monotone_curve_2& xc) const
     { ++m_counter; return m_object(xc); }
   };
 
@@ -429,7 +429,7 @@ class Arr_counting_traits_2 : public BaseTraits {
       m_object(base.compare_endpoints_xy_2_object()), m_counter(counter) {}
 
     /*! operates */
-    Comparison_result operator()(const X_monotone_curve_2& xc)
+    Comparison_result operator()(const X_monotone_curve_2& xc) const
     { ++m_counter; return m_object(xc); }
   };