Functions delegate to Graph.Directed for undirected graph

[achan1989]

Aside from pathfinding issues #37 and #11, several of the Graph functions delegate to Graph.Directed in a way that is inapproriate for undirected graphs:

• is_acyclic?
• components
• strong_components
• reachable
• reachable_neighbors
• reaching
• reaching_neighbors
• preorder
• postorder
• loop_vertices

[tcoopman]

I just ran into this. A basic exampe of what's not working:

> g = Graph.new(type: :undirected) |> Graph.add_edges([{:a, :b}, {:b, :c}])
> Graph.reachable(g, [:c])
[]

But it should return [:a, :b, :c]