Get the directed edges from GO term to GO term.

def _init_edges(p2cs, c2ps):
        """Get the directed edges from GO term to GO term."""
        edge_from_to = []
        for parent, children in p2cs.items():
            for child in children:
                # if child in goids_present and parent in goids_present:
                edge_from_to.append((child, parent))
        for parent, children in c2ps.items():
            for child in children:
                # if child in goids_present and parent in goids_present:
                edge_from_to.append((child, parent))
        return edge_from_to