Add face lists of simplex rings and strings

[MeikeWeiss]

Some more code from Jana's master thesis

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.62%. Comparing base (427e8c7) to head (299b970).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #341      +/-   ##
==========================================
+ Coverage   70.58%   70.62%   +0.03%     
==========================================
  Files          62       62              
  Lines       17269    17290      +21     
==========================================
+ Hits        12190    12211      +21     
  Misses       5079     5079              

Files with missing lines	Coverage Δ
gap/PolygonalComplexes/properties.gd	100.00% <100.00%> (ø)
gap/PolygonalComplexes/properties.gi	68.99% <100.00%> (+0.49%)	⬆️

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