graphIsRooted

In graph theory, "connected components" are really defined only for undirected (ie bidirectional) graphs. Our graphs are directed, because of pruning, but they are mostly undirected. In this case we compute components starting from a single node so what we are really measuring is whether the graph is a "rooted graph". TODO: measure whether the graph is "strongly connected" ie there is a path from every node to every other node.