Working towards running BGL algorithms on ManifoldSurfaceMesh

[afabri]

Hello,
As a mesh is a graph with vertices and edges it is sometimes useful to run graph algorithms on a mesh.
This PR is the first step towards this goal which will enable to write code like

  geometrycentral::surface::VertexData<double> distance(*mesh);

  auto source = *(vertices(*mesh).first);
  boost::breadth_first_search(*mesh, source,
    boost::visitor(boost::make_bfs_visitor(boost::record_distances(boost::make_assoc_property_map(distance), boost::on_tree_edge())))
    .vertex_index_map(boost::associative_property_map<geometrycentral::surface::VertexData<std::size_t>>(mesh->getVertexIndices())));

  for(auto v : mesh->vertices()){
    std::cout << v << " at distance " << distance[v] << std::endl;
  }

[nmwsharp]

looks good, thank you!