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

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.43%. Comparing base (0fe44fd) to head (f7d5b00).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
gap/attr.gi	50.00%	1 Missing ⚠️
gap/planar.gi	75.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (66.66%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #931      +/-   ##
==========================================
- Coverage   97.45%   97.43%   -0.02%     
==========================================
  Files          50       50              
  Lines       21188    21193       +5     
  Branches      639      639              
==========================================
+ Hits        20649    20650       +1     
- Misses        474      478       +4     
  Partials       65       65              

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[james-d-mitchell]

I'm happy with this PR, sorry if it has been sitting here waiting for a review for ages! Only one question: is it really necessary to take the symmetric closure of the graph? In many functions in Digraphs we simply assume the graph is symmetric, and return the answer as if it was. Would this be appropriate here too?

[MeikeWeiss]

Thanks for the input! I changed the handling with symmetric so that it is hopefully more like in other methods.