depth_first_search root vertex

From: Douglas Gregor (gregod_at_[hidden])
Date: 2001-11-28 00:37:33

It can sometimes be useful to start a depth-first search at a specific 
vertex, but not limit the DFS to only nodes reachable from the starting node 
(as would be done by depth_first_visit). 

I suggest making the following syntax work:

---------
  depth_first_search(graph, visitor, color_map, start_vertex);

same as depth_first_search(graph, visitor, color_map), except that 
start_vertex is the first root that the DFS starts from. The old syntax 
stays, of course.

---------
  depth_first_search(graph, root_vertex(start_vertex));

use the root_vertex named parameter to specify the start vertex.

The attached patch against boost/graph/depth_first_search.hpp (CVS version) 
implements the above functionality. Okay to commit this (with appropriate 
documentation?)

        Doug