improving representative frame extraction

[keighrim]

fixes #281.

[keighrim]

@kelleyl this PR might break existing code of yours. Can you review the code and maybe test it (with pip install -e .) against your app code?

[codecov]

Codecov Report

Attention: Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.

Project coverage is 92.48%. Comparing base (af83e13) to head (ab40d2b).
Report is 4 commits behind head on develop.

Files	Patch %	Lines
mmif/utils/video_document_helper.py	80.76%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #291      +/-   ##
===========================================
- Coverage    92.63%   92.48%   -0.15%     
===========================================
  Files           10       10              
  Lines         1384     1398      +14     
===========================================
+ Hits          1282     1293      +11     
- Misses         102      105       +3     

Flag	Coverage Δ
unittests	92.48% <80.76%> (-0.15%)	⬇️

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