- all types should end with _type
- assigning undirected graph to directed creates symmetric graph
	this is an old HACK... is this fixed?
- force_simple is on by default. should be off.

  to get a symmetric directed graph from an undirected, you should do
  undirected_graph_t U(...);
  directed_graph_t D;
  D = U.directed_view(); // <= will assign both directions, naturally
  (this is untested)
- some assignments are inefficient, preallocation is incomplete
- somethings wrong/incomplete with clear_vertex in directed case
- what about oriented/antisymmetric (directed) graphs?
- properties? anybody?
