New rich club metric

[vboussange]

Hi there, I suggest to add the rich-club metric (cf Wikipedia ) to beautiful LightGraphs.jl, under the community metrics.

rich_club(g,k)

Return the non-normalised rich-club coefficient of graph g,
with degree cut-off k.

julia> using LightGraphs
julia> g = star_graph(5)
julia> rich_club(g,1)
0.4

[codecov]

Codecov Report

Merging #1549 (0b9ba32) into master (e7669f2) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 0b9ba32 differs from pull request most recent head 34ad732. Consider uploading reports for the commit 34ad732 to get more accurate results

@@            Coverage Diff             @@
##           master    #1549      +/-   ##
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+ Coverage   99.37%   99.44%   +0.06%     
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  Files         108      107       -1     
  Lines        5157     5560     +403     
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+ Hits         5125     5529     +404     
+ Misses         32       31       -1