Add FFTOperator

[amontoison]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 84.84848% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.35%. Comparing base (6e6bbf8) to head (ea2ae42).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fft_utils.jl	82.75%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #26       +/-   ##
===========================================
+ Coverage   35.32%   47.35%   +12.02%     
===========================================
  Files           9        9               
  Lines        2941     2853       -88     
===========================================
+ Hits         1039     1351      +312     
+ Misses       1902     1502      -400     

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