BGL graphviz output without attributes

From: Douglas Gregor (gregod_at_[hidden])
Date: 2001-12-06 15:42:56

Hello,
  Currently, the BGL's graphviz output requires graph, vertex, and edge 
attributes. Since these attributes are not required by the graphviz format 
itself, they probably should not be required by BGL's writer, either.
  The attached patch makes all of these attributes optional. Okay to commit?

        Doug