Rename has_diagonal into augmented_graph

[amontoison]

close #272

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (33a1ef1) to head (8ea6759).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #273   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         1997      1997           
=========================================
  Hits          1997      1997           

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[gdalle]

One small clarification, otherwise this is good to go. Thanks for splitting #267 into more palatable chunks!