Use cycle methods of Digraphs

[MeikeWeiss]

I removed our own cycle methods and replaced them by new cycle methods implemented in Digraphs. This probably works only after PR#702 of Digraphs is merged.

TODO

[codecov]

Codecov Report

Attention: Patch coverage is 82.69231% with 9 lines in your changes missing coverage. Please review.

Project coverage is 76.36%. Comparing base (25baabb) to head (94c6943).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/PolygonalComplexes/graphs.gi	84.37%	5 Missing ⚠️
gap/Paths/paths.gi	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
- Coverage   76.73%   76.36%   -0.37%     
==========================================
  Files          61       61              
  Lines       16381    16190     -191     
==========================================
- Hits        12570    12364     -206     
- Misses       3811     3826      +15     

Files with missing lines	Coverage Δ
gap/Paths/paths.gd	100.00% <ø> (ø)
gap/PolygonalComplexes/graphs.gd	100.00% <ø> (ø)
unit_tests/Test_Graphs.g	100.00% <100.00%> (ø)
unit_tests/test_main.g	97.61% <100.00%> (+0.11%)	⬆️
gap/Paths/paths.gi	79.45% <50.00%> (-0.13%)	⬇️
gap/PolygonalComplexes/graphs.gi	94.91% <84.37%> (-2.75%)	⬇️

[MeikeWeiss]

I have changed the tests so that the bug fixed in PR#702 in digraphs should not matter. So I think this can theoretically be merged.