Added clusters_from_cover to kmapper.

[pulquero]

2nd part for #208.

[codecov]

Codecov Report

Merging #213 (2349b6b) into master (6d983e6) will increase coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   78.42%   78.80%   +0.38%     
==========================================
  Files          11       11              
  Lines         774      788      +14     
  Branches      159      164       +5     
==========================================
+ Hits          607      621      +14     
  Misses        138      138              
  Partials       29       29              

Impacted Files	Coverage Δ
kmapper/cover.py	88.42% <100.00%> (+0.92%)	⬆️
kmapper/kmapper.py	88.84% <100.00%> (+0.30%)	⬆️

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[sauln]

#214 subsumes this PR.