Move SL_n code by Akos and Max into separate file

[fingolfin]

This further reduces merge issues with Daniel's branch

[codecov]

Codecov Report

❌ Patch coverage is 82.92683% with 98 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.01%. Comparing base (8075789) to head (c49a15e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/projective/sl.gi	82.92%	98 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #397   +/-   ##
=======================================
  Coverage   73.01%   73.01%           
=======================================
  Files          43       44    +1     
  Lines       18370    18370           
=======================================
  Hits        13412    13412           
  Misses       4958     4958           

Files with missing lines	Coverage Δ
gap/projective/classicalnatural.gi	91.79% <ø> (+3.93%)	⬆️
gap/projective/sl.gi	82.92% <82.92%> (ø)

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