fix stiefelexp

[Jutho]

This should fix #18 . Maybe a small review by @lkdvos or @KVerv ?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.63%. Comparing base (6ed66c1) to head (d281f3b).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   72.57%   72.63%   +0.05%     
==========================================
  Files           5        5              
  Lines         485      486       +1     
==========================================
+ Hits          352      353       +1     
  Misses        133      133              

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[Jutho]

Also, after this PR is merged, I consider moving this to QuantumKitHub and changing the formatting to Runic etc.

[lkdvos]

Overall looks reasonable to me, left a minor comment but I don't think this matters all that much