Fix SL godown eigenspace dimension check

[fingolfin]

This prevents non-invariant spaces from reaching LinearAction.

Co-authored-by: Codex codex@openai.com

[fingolfin]

This fixes a very rare error, but it occurred in PR #512 (due to bad luck).

Codex came up with a slightly different "fix" first, but after some back and forth, we settled on this. I think the main difference is that before, it could happen that all eigenspaces have dimension 1, but we really also want an eigenspace of dimension 3 (when d=4, which is the only case we actually use, I believe).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (f6b57ad) to head (607d771).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #516   +/-   ##
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  Coverage   86.51%   86.51%           
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  Files          45       45           
  Lines       19262    19262           
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  Hits        16665    16665           
  Misses       2597     2597           

Files with missing lines	Coverage Δ
gap/projective/sl.gi	98.41% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[Till-Eisen]

I could see the case $d-1 = dim-d$ as a problem in the previous code. In this case there might have been "to many" eigenspaces of dimension 1.
Perhaps $GL(7,8)$ instead of $GL(6,8)$ would be a good test as well, since then $dim=7, d= 4$ and thereby $dim-d=d-1$?

[fingolfin]

In fact also $dim=5$ is of interest, too, as then $dim-d=1$