Filter track buffers

[emdavis02]

Issue #10 and handling frames with only one instance

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 6 lines in your changes missing coverage. Please review.

Project coverage is 65.89%. Comparing base (b034eb7) to head (fdd24e8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sleap_mot/tracker.py	53.84%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   65.76%   65.89%   +0.13%     
==========================================
  Files           8        8              
  Lines         552      560       +8     
==========================================
+ Hits          363      369       +6     
- Misses        189      191       +2     

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