Return True if node u has neighbor v.
This returns True if there exists any edge (u,v,data) for some data.
See also
Examples
>>> G=nx.path_graph(4)
>>> G.has_neighbor(0,1)
True
>>> G.has_edge(0,1) # same as has_neighbor
True
>>> 1 in G[0] # this gives KeyError if u not in G
True