frozen graph
------------
>>> import networkx as nx
>>> G=nx.star_graph(2)
>>> nx.is_frozen(G)
False
>>> G=nx.freeze(G)
>>> nx.is_frozen(G)
True
>>> G.add_node(1)
Traceback (most recent call last):
...
NetworkXError: Frozen graph can't be modified
>>> G.add_edge(1,2)
Traceback (most recent call last):
...
NetworkXError: Frozen graph can't be modified
