Fix sanity checks for Eulerian trail

[evanfields]

The sanity check for a graph having an Eulerian trail was incorrect: for a connected simple graph to have an Eulerian trail there must be exactly two odd degree vertices, but the existing code checks for exactly two even degree vertices. The current test happened to test on a 4-vertex graph with two even and two odd degree vertices, so the test passed.

I just flipped the check and added the relevant tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.32%. Comparing base (aa3d197) to head (8f65af2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
+ Coverage   97.31%   97.32%   +0.01%     
==========================================
  Files         120      120              
  Lines        6953     6953              
==========================================
+ Hits         6766     6767       +1     
+ Misses        187      186       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[gdalle]

thank you!