add Less_x_2 and Less_y_2 in the traits concept and its models

this is needed to use the spatial sorting

Segment_Delaunay_graph_2/doc/Segment_Delaunay_graph_2/Concepts/SegmentDelaunayGraphTraits_2.h

@@ -148,6 +148,18 @@ represented by the sites `s1` and `s2`.
 */ 
 typedef unspecified_type Compare_y_2; 
 
+/*!
+A predicate object type. Must
+provide `bool operator()(Point_2 p1, Point_2 p2)`, which returns `true` if `p1.x() < p2.x()`.
+*/
+typedef unspecified_type Less_x_2;
+
+/*!
+A predicate object type. Must
+provide `bool operator()(Point_2 p1, Point_2 p2)`, which returns `true` if `p1.y() < p2.y()`.
+*/
+typedef unspecified_type Less_y_2;
+
 /*! 
 A predicate object type. Must 
 provide `Orientation operator()(Site_2 s1, Site_2 s2, Site_2 s3)`, which performs the 
@@ -305,6 +317,16 @@ Compare_y_2 compare_y_2_object();
 
 /*!
 
+*/
+Less_x_2 less_x_2_object();
+
+/*!
+
+*/
+Less_y_2 less_y_2_object();
+
+/*! 
+
 */ 
 Orientation_2 orientation_2_object(); 
 

Segment_Delaunay_graph_2/include/CGAL/Segment_Delaunay_graph_2/Filtered_traits_base_2.h

@@ -335,6 +335,9 @@ public:
 		     FK_Is_degenerate_edge_2, C2E, C2F>
   Is_degenerate_edge_2;
 
+  typedef typename CK::Less_x_2 Less_x_2;
+  typedef typename CK::Less_y_2 Less_y_2;
+
 private:
   typedef
   Filtered_predicate<EK_Arrangement_type_2, FK_Arrangement_type_2, C2E, C2F>
@@ -448,6 +451,16 @@ public:
   oriented_side_2_object() const {
     return Oriented_side_2();
   }
+
+  Less_x_2
+  less_x_2_object() const {
+    return Less_x_2();
+  }
+
+  Less_y_2
+  less_y_2_object() const {
+    return Less_y_2();
+  }
 };
 
 

Segment_Delaunay_graph_2/include/CGAL/Segment_Delaunay_graph_2/Traits_base_2.h

@@ -171,6 +171,9 @@ public:
   CGAL_SEGMENT_DELAUNAY_GRAPH_2_NS::Oriented_side_C2<K,MTag>
   Oriented_side_2;
 
+  typedef typename R::Less_x_2 Less_x_2;
+  typedef typename R::Less_y_2 Less_y_2;
+
 public:
   //-----------------------------------------------------------------------
   //                  ACCESS TO OBJECTS
@@ -264,6 +267,15 @@ public:
     return Oriented_side_2();
   }
 
+  Less_x_2
+  less_x_2_object() const {
+    return Less_x_2();
+  }
+
+  Less_y_2
+  less_y_2_object() const {
+    return Less_y_2();
+  }
 };
 
 } //namespace CGAL