Incorporating video document helper fixes

[kelleyl]

This PR incorporates the feature request in #345 with the bug fixes to the video_document_helper.py module.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.95%. Comparing base (978e389) to head (a6b1982).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
mmif/utils/video_document_helper.py	88.88%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #373      +/-   ##
===========================================
+ Coverage    73.76%   73.95%   +0.18%     
===========================================
  Files           22       22              
  Lines         3023     3048      +25     
===========================================
+ Hits          2230     2254      +24     
- Misses         793      794       +1     

Flag	Coverage Δ
unittests	73.95% <88.88%> (+0.18%)	⬆️

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