added Chinese Whisphers clustering

[oxinabox]

This adds Chinese Whisphers clustering. It works, the code could no doubt be better.

I've not done any docs yet. Here is the paper it is from: https://pdfs.semanticscholar.org/3e71/0251cb01ba6e1c0c735591776a212edc461f.pdf

This won't work until https://github.com/JuliaCollections/DataStructures.jl/pull/375 is merged and tagged.

It use the abstractions I suggested in https://github.com/JuliaLang/julia/issues/26613#issuecomment-376040464 This kinda spare matrix handling is I think a good for graph clustering algorithms

[ararslan]

Looks like you haven't included the file here.

[oxinabox]

Oops, fixed.