Document graph topologies, implement layer over MPI_Topo_test

[SVN r37853]

include/boost/mpi/communicator.hpp

@@ -766,6 +766,21 @@ class communicator
    */
   communicator split(int color, int key) const;
 
+  /**
+   * Determines whether this communicator has a Cartesian topology.
+   */
+  bool has_cartesian_topology() const;
+
+  /**
+   *  Determines whether this communicator has a graph topology. If
+   *  the communicator does have a graph topology, the communicator
+   *  itself can be viewed as a graph by the algorithms in the Boost
+   *  Graph Library. The communicator then meets the requirements of
+   *  the @c Graph, @c Incidence Graph, @c Adjacency Graph, @c Vertex
+   *  List Graph, and @c Edge List Graph concepts.
+   */
+  bool has_graph_topology() const;
+
   /**
    *  Create a new communicator whose topology is described by the
    *  given graph. The vertex index map (@p index) gives the mapping

include/boost/mpi/graph_topology.hpp

@@ -377,6 +377,10 @@ struct graph_traits<mpi::communicator> {
   { 
   };
 
+  /**
+   * @brief Returns a vertex descriptor that can never refer to any
+   * valid vertex.
+   */
   static vertex_descriptor null_vertex() { return -1; }
 
   // Incidence Graph requirements
@@ -396,6 +400,10 @@ struct graph_traits<mpi::communicator> {
 };
 
 // Property Graph requirements
+
+/**
+ * INTERNAL ONLY
+ */
 template<>
 struct property_map<mpi::communicator, vertex_index_t>
 {