LEDA graph adaptors do not handle hidden nodes properly

[Douglas Gregor]

Hidden nodes are completely ignored by the LEDA graph adaptors. 
This may cause problems when, for instance, there are hidden 
nodes but the equivalent of vertex_index is used to construct a 
(BGL) property map. The LEDA adaptors should probably take the 
same approach as filtered_graph: let num_vertices() return the 
number of the nodes in the graph plus the number of hidden nodes, 
but vertices() filters out the hidden nodes. Similarly for edges.