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 56.84211% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.24%. Comparing base (978e389) to head (09c7e32).
⚠️ Report is 11 commits behind head on develop.

Files with missing lines	Patch %	Lines
mmif/utils/video_document_helper.py	56.84%	41 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## develop #373 +/- ##
===========================================
- Coverage 73.76% 73.24% -0.53% 
===========================================
Files 22 22 Lines 3023 3106 +83 ===========================================
+ Hits 2230 2275 +45 - Misses 793 831 +38 

Flag	Coverage Δ
unittests	73.24% <56.84%> (-0.53%)	⬇️

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