add RECOG.FindSL2inSL3, i.e. find a subgroup isomorphic to SL(2,q) of G isomorphic to SL(3,q).

[Till-Eisen]

Given G isomorphic to SL(3,q) and (odd) q, finds a subgroup that is almost always isomorphic to SL(2,q).

[Till-Eisen]

Perhaps one could implement a loop in the main function that only returns a group until it (almost) surely is isomorphic to SL(2,q).

And also tests should be added.

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.32%. Comparing base (858e2e7) to head (0ca26f7).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/projective/sl3_natural.gi	16.66%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage   88.46%   88.32%   -0.14%     
==========================================
  Files          45       46       +1     
  Lines       19019    19055      +36     
==========================================
+ Hits        16825    16831       +6     
- Misses       2194     2224      +30     

Files with missing lines	Coverage Δ
gap/projective/sl3_natural.gi	16.66% <16.66%> (ø)

... and 1 file with indirect coverage changes

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