fix infinite loop in cond

[araujoms]

Closes #680

For n==2 this code was trying to find three non-parallel +-1 vectors, which is mathematically impossible.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.37%. Comparing base (2b32b60) to head (cad723f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #682      +/-   ##
==========================================
+ Coverage   84.28%   84.37%   +0.09%     
==========================================
  Files          13       13              
  Lines        9340     9340              
==========================================
+ Hits         7872     7881       +9     
+ Misses       1468     1459       -9     

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[JamesWrigley]

Weak LGTM!

(I have a tenuous understanding of the problem, but the logic and new tests make sense to me)

[araujoms]

I can't say I fully understand opnormestinv either. I'm certain my fix doesn't affect the previously working cases, and improves the situation for n == 2, so I'm comfortable with merging.

Nevertheless, it would be better if somebody who does understand it could comment. I did some archaeology, and who originally wrong this function was @mfasi, maybe he'll spare us a glance?