Clean up sample_rate_offset() and frequency_offset()

[mhostetter]

Fixes #343

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 27.27273% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 79.33%. Comparing base (1e1bb5a) to head (666ff3e).

Files	Patch %	Lines
src/sdr/_simulation/_impairment.py	20.00%	8 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           release/0.0.x     #368      +/-   ##
=================================================
- Coverage          79.40%   79.33%   -0.07%     
=================================================
  Files                 63       63              
  Lines               3782     3770      -12     
=================================================
- Hits                3003     2991      -12     
  Misses               779      779              

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