bug fix in ConcatenationPaths

[aniemeyer]

There is a bug in ConcatenationOfPaths whereby two paths are to be concatenated. The use of Union created a set and not an ordered list, which leads to a bug.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.20%. Comparing base (113f0df) to head (053a219).
Report is 16 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
- Coverage   78.30%   78.20%   -0.11%     
==========================================
  Files          61       61              
  Lines       16942    16969      +27     
==========================================
+ Hits        13266    13270       +4     
- Misses       3676     3699      +23     

Files with missing lines	Coverage Δ
gap/Paths/paths.gi	84.77% <100.00%> (ø)

... and 1 file with indirect coverage changes