Return communicability between all pairs of nodes in G.
Communicability between pair of node (u,v) of node in G is the sum of closed walks of different lengths starting at node u and ending at node v.
Parameters :  G: graph : 

Returns :  comm: dictionary of dictionaries :

Raises :  NetworkXError :

See also
Notes
This algorithm uses matrix exponentiation of the adjacency matrix.
Let G=(V,E) be a simple undirected graph. Using the connection between the powers of the adjacency matrix and the number of walks in the graph, the communicability between nodes u and v is [R140],
where is the adjacency matrix of G.
References
[R140]  (1, 2) Ernesto Estrada, Naomichi Hatano, “Communicability in complex networks”, Phys. Rev. E 77, 036111 (2008). http://arxiv.org/abs/0707.0756 
Examples
>>> G = nx.Graph([(0,1),(1,2),(1,5),(5,4),(2,4),(2,3),(4,3),(3,6)])
>>> c = nx.communicability_exp(G)