Compute closeness centrality for nodes.
Closeness centrality [R157] of a node is the reciprocal of the sum of the shortest path distances from to all other nodes. Since the sum of distances depends on the number of nodes in the graph, closeness is normalized by the sum of minimum possible distances .
where is the shortestpath distance between and , and is the number of nodes in the graph.
Notice that higher values of closeness indicate higher centrality.
Parameters :  G : graph
u : node, optional
distance : edge attribute key, optional (default=None)
normalized : bool, optional


Returns :  nodes : dictionary

Notes
The closeness centrality is normalized to where is the number of nodes in the connected part of graph containing the node. If the graph is not completely connected, this algorithm computes the closeness centrality for each connected part separately.
If the ‘distance’ keyword is set to an edge attribute key then the shortestpath length will be computed using Dijkstra’s algorithm with that edge attribute as the edge weight.
References
[R157]  (1, 2) Freeman, L.C., 1979. Centrality in networks: I. Conceptual clarification. Social Networks 1, 215–239. http://www.soc.ucsb.edu/faculty/friedkin/Syllabi/Soc146/Freeman78.PDF 