Return the density of a graph.
The density for undirected graphs is
d = \frac{2m}{n(n-1)},
and for directed graphs is
d = \frac{m}{n(n-1)},
where n is the number of nodes and m is the number of edges in G.
Notes
The density is 0 for an graph without edges and 1.0 for a complete graph.
The density of multigraphs can be higher than 1.