fix xmax for Fourier operators

[ranocha]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.70%. Comparing base (360c87f) to head (b2df85a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #256      +/-   ##
==========================================
+ Coverage   90.69%   90.70%   +0.01%     
==========================================
  Files          33       33              
  Lines        5106     5112       +6     
==========================================
+ Hits         4631     4637       +6     
  Misses        475      475              

Flag	Coverage Δ
unittests	90.70% <100.00%> (+0.01%)	⬆️

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[coveralls]

coverage: 91.609% (+0.009%) from 91.6%
when pulling b2df85a on hr/xmax_fourier
into 360c87f on main.