Triangulation_2: Document degree() (#8976)

## Summary of Changes

Add the documentation of `degree()`  and also of `is_vertex()`. 

## Release Management

* Affected package(s): 2D Triangulations
* Issue(s) solved (if any): fix #8975
* License and copyright ownership: unchanged

Periodic_3_triangulation_3/doc/Periodic_3_triangulation_3/CGAL/Periodic_3_triangulation_3.h

@@ -1312,7 +1312,7 @@ OutputIterator
 adjacent_vertices(Vertex_handle v, OutputIterator vertices) const;
 
 /*!
-Returns the degree of a vertex, that is, the number of adjacent vertices.
+Returns the degree of `v`, that is, the number of adjacent vertices.
 \pre `v` \f$ \neq\f$ `Vertex_handle()`, `t`.`is_vertex(v)`.
 */
 size_type degree(Vertex_handle v) const;

TDS_3/doc/TDS_3/Concepts/TriangulationDataStructure_3.h

@@ -960,7 +960,7 @@ OutputIterator
 adjacent_vertices(Vertex_handle v, OutputIterator vertices) const;
 
 /*!
-Returns the degree of a vertex, that is, the number of incident vertices.
+Returns the degree of `v`, that is, the number of incident vertices.
 \pre `v` \f$ \neq\f$ `Vertex_handle()`, `tds.is_vertex(v)`.
 */
 size_type degree(Vertex_handle v) const;

Triangulation_2/doc/Triangulation_2/CGAL/Triangulation_2.h

@@ -472,7 +472,7 @@ const TriangulationDataStructure_2 & tds() const;
 
 /// @}
 
-/// \name Non const access
+/// \name Non-Const Access
 /// \attention The responsibility of keeping a valid triangulation belongs to the
 /// user when using advanced operations allowing a direct manipulation
 /// of the `tds`. This method is mainly a help for users implementing
@@ -528,6 +528,11 @@ is_infinite(Edge_circulator ec) const;
 bool
 is_infinite(All_edges_iterator ei) const;
 
+/*!
+`true` if `v` is a vertex of the triangulation.
+*/
+bool is_vertex(Vertex_handle v) const;
+
 /*!
 `true` if there is an edge having `va` and `vb` as
 vertices.
@@ -1117,6 +1122,13 @@ in counterclockwise order around `v`.
 */
 Vertex_circulator incident_vertices(Vertex_handle v, Face_handle f) ;
 
+/*!
+Returns the degree of `v`, that is, the number of incident vertices.
+The infinite vertex is counted.
+\pre `v != Vertex_handle()`, `t.is_vertex(v)`.
+*/
+size_type degree(Vertex_handle v) const;
+
 /// @}
 
 /// \name Traversal Between Adjacent Faces

Triangulation_2/include/CGAL/Triangulation_2.h

@@ -294,6 +294,7 @@ public:
   bool is_infinite(const Edge& e) const;
   bool is_infinite(const Edge_circulator& ec) const;
   bool is_infinite(const All_edges_iterator& ei) const;
+  bool is_vertex(Vertex_handle va) const;
   bool is_edge(Vertex_handle va, Vertex_handle vb) const;
   bool is_edge(Vertex_handle va, Vertex_handle vb, Face_handle& fr,
                int & i) const;
@@ -1096,6 +1097,14 @@ is_infinite(const All_edges_iterator& ei) const
   return is_infinite(*ei);
 }
 
+template <class Gt, class Tds >
+inline bool
+Triangulation_2<Gt, Tds>::
+is_vertex(Vertex_handle va) const
+{
+  return _tds.is_vertex(va);
+}
+
 template <class Gt, class Tds >
 inline bool
 Triangulation_2<Gt, Tds>::

Triangulation_2/test/Triangulation_2/include/CGAL/_test_cls_triangulation_2.h

@@ -182,6 +182,7 @@ _test_cls_triangulation_2( const Triangul & )
   Triangul T0_2;
   Vertex_handle v0_2_0 =   T0_2.insert_first(p0);
   assert( v0_2_0 != nullptr );
+  assert( T0_2.is_vertex(v0_2_0) );
   assert( T0_2.dimension() == 0 );
   assert( T0_2.number_of_vertices() == 1 );
   assert( T0_2.number_of_faces() == 0);

Triangulation_3/doc/Triangulation_3/CGAL/Triangulation_3.h

@@ -1669,7 +1669,7 @@ OutputIterator
 finite_adjacent_vertices(Vertex_handle v, OutputIterator vertices) const;
 
 /*!
-Returns the degree of a vertex, that is, the number of incident vertices.
+Returns the degree of `v`, that is, the number of incident vertices.
 The infinite vertex is counted.
 \pre `v != Vertex_handle()`, `t.is_vertex(v)`.
 */