Fixed issue with leaky indices

[fronzbot]

• On certain inputs, the find_harmonics function would grab an index right at nfft/2 rather than aliasing it back down to 0.
• Fix added with appropriate tests to check that situation

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.68%. Comparing base (d637c9d) to head (98269f7).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #40      +/-   ##
==========================================
+ Coverage   65.82%   67.68%   +1.86%     
==========================================
  Files           4        4              
  Lines         158      164       +6     
==========================================
+ Hits          104      111       +7     
+ Misses         54       53       -1     

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