Add constructive recognition for SL_n, take 2

[fingolfin]

This is a successor for PR #330. The only difference for now is that the branch is now hosted in this repo, not in @danielrademacher's branch. This will make it easier for us here to work on it together.

The plan is to further reduce diffs between this and master, and then get it merged.

There will also soon be another PR / branch with the rest of Daniel's code.

[codecov]

Codecov Report

❌ Patch coverage is 40.43439% with 2002 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.71%. Comparing base (4fecba4) to head (891fb8c).

Files with missing lines	Patch %	Lines
...projective/constructive_recognition/utils/utils.gi	19.53%	902 Missing ⚠️
...rojective/constructive_recognition/SL/GoingDown.gi	19.65%	376 Missing ⚠️
...ective/constructive_recognition/SL/sl2_BlackBox.gi	8.91%	327 Missing ⚠️
...projective/constructive_recognition/SL/BaseCase.gi	17.62%	243 Missing ⚠️
gap/projective/constructive_recognition/SL/main.gi	40.14%	85 Missing ⚠️
...ojective/constructive_recognition/utils/achieve.gi	90.98%	61 Missing ⚠️
.../projective/constructive_recognition/SL/GoingUp.gi	97.30%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   71.55%   65.71%   -5.84%     
==========================================
  Files          43       49       +6     
  Lines       18399    20457    +2058     
==========================================
+ Hits        13165    13444     +279     
- Misses       5234     7013    +1779     

Files with missing lines	Coverage Δ
gap/base/kernel.gi	81.66% <100.00%> (+0.31%)	⬆️
.../projective/constructive_recognition/SL/GoingUp.gi	97.30% <97.30%> (ø)
...ojective/constructive_recognition/utils/achieve.gi	90.98% <90.98%> (ø)
gap/projective/constructive_recognition/SL/main.gi	40.14% <40.14%> (ø)
...projective/constructive_recognition/SL/BaseCase.gi	17.62% <17.62%> (ø)
...ective/constructive_recognition/SL/sl2_BlackBox.gi	8.91% <8.91%> (ø)
...rojective/constructive_recognition/SL/GoingDown.gi	19.65% <19.65%> (ø)
...projective/constructive_recognition/utils/utils.gi	19.53% <19.53%> (ø)

... and 4 files with indirect coverage changes

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