VDH image extractions now return source timepoints

[keighrim]

fixes#384

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.76%. Comparing base (cb6f952) to head (23f8d59).

Files with missing lines	Patch %	Lines
mmif/utils/video_document_helper.py	87.50%	18 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## develop #385 +/- ##
===========================================
+ Coverage 73.62% 74.76% +1.14% 
===========================================
Files 27 27 Lines 3204 3226 +22 ===========================================
+ Hits 2359 2412 +53 + Misses 845 814 -31 

Flag	Coverage Δ
unittests	74.76% <87.50%> (+1.14%)	⬆️

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