topo: make new minimal Graph object a Graph, not a DiGraph

Fixes another Graph regression relative to NetworkX.

RipL broke because the NetworkX Graph object that was used previously
for topologies is an undirected graph:

>>> import networkx as nx
>>> g=nx.Graph()
>>> g.add_edge(0,1)
>>> g[1]
{0: {}}
>>> g[0]
{1: {}}

There is a separate DiGraph object in NetworkX for directed behavior.

The minimal replacement previously implemented DiGraph behavior.

>>> from mininet.topo import Graph
>>> g2=Graph()
>>> g2.add_edge(0,1)
>>> g2[0]
[1]
>>> g2[1]
[]

This commit restores undirected graph behavior.