Upstream controller recursive implementation

[ddomingof]

Given a BELGraph and a node, find all of the upstream controllers:

controllers(g, n) = set.union(controllers(g, u) for u upstream of n in g)

[cthoyt]

@ewollert this would be a cool thing to work towards at the end of march. We'll enrich some graphs we have with drugs/chemicals and apply this function to ask the question: which molecules increase (or decrease) a given node?