extract sub graph from GO graph, the sub graph contains a set of target
go terms , and related in the pathway from the root to its leaf nodes, for
example, one pathway is A->B->C->Target->D->E->F, while A is root and F is
leaf, and suppose parentLevel and childLevel are both 1, then the nodes C,
Taraget, and D are extracted intio the subGraph, the the related edges are
extracted into the subGraph too.