Fix size issue in SpStabilizerOfIsotropicSubspace

[TristanPfersdorff]

Pretty much everything I mentioned in here, tackling #127.

[fingolfin]

This looks but the base CI is badly broken. Ideally this is fixed first and then we merge this PR when its CI passes... I'll see what I can do

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.51%. Comparing base (f569bac) to head (9a534da).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #128   +/-   ##
=======================================
  Coverage   94.51%   94.51%           
=======================================
  Files          14       14           
  Lines        4682     4684    +2     
=======================================
+ Hits         4425     4427    +2     
  Misses        257      257           

Files with missing lines	Coverage Δ
gap/ReducibleMatrixGroups.gi	98.75% <100.00%> (+<0.01%)	⬆️

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