Change unit conversion to work with inverse units (e.g. frequencies)

[iguinn]

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[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.51%. Comparing base (f7780ec) to head (641e1cb).
Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dspeed/processing_chain.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
- Coverage   60.53%   60.51%   -0.02%     
==========================================
  Files          48       48              
  Lines        3233     3234       +1     
==========================================
  Hits         1957     1957              
- Misses       1276     1277       +1     

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