Warn about directed undirected graphs

[jakobnordstrom]

I think it would be a very nice and helpful feature of CNFgen if, when the program is expecting an undirected (non-bipartite) graph but gets a directed one, it would issue a gentle warning about this.

It might still be natural to use the semantics that if v is mentioned as a neighbour of u, then (u, v) is an undirected edge, and to generate whatever the formula is. But as a graph generator programmer I know that I want also to have u mentioned in the neighbour list of v in this case, and if I screwed up then it would be extremely helpful to have someone alert me to this.