title: indexing vertices in polyhedron_3 class
body: <p>I am new to CGAL. I know we can use vertex iterator to iterate all vertices in a polyhedron object. But how can I randomly sample a vertex in a given polyhedron and do something around it.  </p>
  
  <p>And does CGAL provide any way to index the 3D vertices? Like we use [x,y](integers) to index pixels in a image.</p>