try to make get_span() work for cyclic graphs

[arne-cl]

• Conano graphs are slightly cyclic
• there are ways to do a topological sort for cyclic graphs

http://stackoverflow.com/questions/17148103/topological-sort-of-cyclic-graph-with-minimum-number-of-violated-edges http://en.wikipedia.org/wiki/Feedback_arc_set

• feedback arc sets are implemented in Sage

http://www.sagemath.org/doc/reference/graphs/sage/graphs/digraph.html#sage.graphs.digraph.DiGraph.feedback_edge_set

[arne-cl]

We're just talking about one conano file:

maz-8361: contains wrong Conano spans maximum recursion depth exceeded