is.cylic is bugged

[tzimiskes]

currently, is.cylic only really works when the edge set of the input is directed.

doing meek rules may help with this situation, but I suspect the topological sort algorithm may need need a slight rethink/

test case:
the edge list:
X1 X2 ---
X2 X3 ---
X3 X1 ---

should (need to check) be incorrectly labeled as "cyclic"

[tzimiskes]

I think the bug is resolved by ignoring undirected edges in pdags. need to talk to Erich to confirm this

[tzimiskes]

Closed pending tests