Get the directed edges from GO term to GO term.

def get_edges(self):
        """Get the directed edges from GO term to GO term."""
        edge_from_to = []
        for parent, children in self.p_from_cs.items():
            for child in children:
                edge_from_to.append((child, parent))
        for parent, children in self.c_from_ps.items():
            for child in children:
                edge_from_to.append((child, parent))
        return edge_from_to