Add graph6

[GroteGnoom]

Lots of work still to be done, like directed graphs, but also cleanup.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.51%. Comparing base (4883cba) to head (bbaa971).
Report is 154 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2526      +/-   ##
===========================================
+ Coverage    84.43%   84.51%   +0.08%     
===========================================
  Files          374      375       +1     
  Lines        60632    60599      -33     
  Branches         0    11825   +11825     
===========================================
+ Hits         51194    51215      +21     
+ Misses        9438     9384      -54     

Files	Coverage Δ
src/io/nauty.c	100.00% <100.00%> (ø)

... and 137 files with indirect coverage changes

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