add RECOG.FindSL2inSL4: Find SL(2,q) as stingray subgroup of SL(4,q)

[Till-Eisen]

This PR adds gap/projective/SL4_natural.gi, which implements RECOG.FindSL2inSL4(G, N).

Given G = SL(4,q) (q odd), the function finds a subgroup U isomorphic to SL(2,q) stingray-embedded in G: concretely, a base change matrix bas is computed such that every element of U is block-diagonal diag(A, I_2) in the new basis.

[codecov]

Codecov Report

❌ Patch coverage is 90.19608% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.67%. Comparing base (858e2e7) to head (ec5a52c).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/projective/sl4_natural.gi	90.19%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
+ Coverage   88.46%   88.67%   +0.21%     
==========================================
  Files          45       46       +1     
  Lines       19019    19070      +51     
==========================================
+ Hits        16825    16911      +86     
+ Misses       2194     2159      -35     

Files with missing lines	Coverage Δ
gap/projective/sl4_natural.gi	90.19% <90.19%> (ø)

... and 3 files with indirect coverage changes

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[fingolfin]

Good, now that your functions gets loaded, have you tried actually using it, i.e., by modifying RECOG.SLn_constructsl2 to call you code, instead?