Fix direction handling and incorrect EulerianDigraph requirement in FacialWalks

[MeikeWeiss]

I fixed the issue #930 by really ignoring the direction of the edges. Additionally I fixed the same for FacialWalks and clarified the documentation of DualPlanarGraph. While doing this I realized that FacialWalks requires an EulerianDigraph as an input which is not correct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.45%. Comparing base (0fe44fd) to head (417cdcc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #931      +/-   ##
==========================================
- Coverage   97.45%   97.45%   -0.01%     
==========================================
  Files          50       50              
  Lines       21188    21186       -2     
  Branches      639      639              
==========================================
- Hits        20649    20646       -3     
- Misses        474      475       +1     
  Partials       65       65              

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