Add min_snr()

[mhostetter]

Fixes #316

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 35.48387% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 78.57%. Comparing base (9de5494) to head (b4e5d6a).

Files	Patch %	Lines
src/sdr/_detection/_theory.py	35.48%	20 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           release/0.0.x     #329      +/-   ##
=================================================
- Coverage          78.98%   78.57%   -0.41%     
=================================================
  Files                 59       59              
  Lines               3645     3669      +24     
=================================================
+ Hits                2879     2883       +4     
- Misses               766      786      +20     

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