Given a DAG, return a pydot digraph object.

def get_pydot_graph(self):
        """Given a DAG, return a pydot digraph object."""
        rel = "is_a"
        # Initialize empty dag
        dag = pydot.Dot(graph_type='digraph', **self.kws['dag'])
        # Initialize nodes
        go2node = self._get_go2pydotnode()
        # Add nodes to graph
        for node in go2node.values():
            dag.add_node(node)
        # Add edges to graph
        rel2edgekws = self.rel2edgekws
        self.edgesobj.chk_edges()
        edgekws = rel2edgekws.get(rel)
        self._add_edges(self.edgesobj.edges, go2node, dag, **edgekws)
        for reltype, edges_list in self.edgesobj.edges_rel.items():
            edgekws = rel2edgekws.get(reltype)
            self._add_edges(edges_list, go2node, dag, **edgekws)
        return dag