Add `mycielski` operator

[mcognetta]

This adds the Mycielski operator (wiki), a unary graph operator that increases the chromatic number of a graph by 1.

This can be found in other graph libraries: NetworkX, SageMath.

I wasn't sure if it was best to put it here or in the generators module (which is how it is implemented in NetworkX/SageMath). Happy to move it if necessary.

[mcognetta]

@aurorarossi thanks for the comments!

[codecov[bot]]

Codecov Report

Merging #177 (fa6c65d) into master (1783617) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #177   +/-   ##
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  Coverage   97.47%   97.48%           
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  Files         109      109           
  Lines        6349     6366   +17     
=======================================
+ Hits         6189     6206   +17     
  Misses        160      160           

[simonschoelly]

This is an interesting operator, I added a few suggestions.