New s_metric method

[vboussange]

Hi there, I propose to add a new metric, so called s-metric.
This is also done in networkX.

s_metric(g; norm=true)

Return the normalised s-metric of g.
The s-metric is defined as the sum of the product of degrees between pair of nodes
for every edge in g. Ref
It is normalised by the maximum s_metric obtained from the family of graph
with similar degree distribution.
s_max is computed from an approximation formula as in https://journals.aps.org/pre/pdf/10.1103/PhysRevE.75.046102
If norm=false, no normalisation is performed.

Examples

julia> using LightGraphs
julia> s_metric(star_graph(4))
0.6

[codecov]

Codecov Report

Merging #1548 (0de03f8) into master (e7669f2) will increase coverage by 0.06%.
The diff coverage is 100.00%.

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