adding utils for handling VideoDocument

[keighrim]

fixes #221
fixes #229

This PR adds extra dependency installation in the build process, and based on those CV-related dependencies, add mmif.utils package with helpers for handing annotations on time-based media (mostly VideoDocument).

[codecov]

Codecov Report

Merging #233 (8c17fa1) into develop (89bbd15) will decrease coverage by 2.21%.
The diff coverage is 68.51%.

@@             Coverage Diff             @@
##           develop     #233      +/-   ##
===========================================
- Coverage    94.30%   92.09%   -2.21%     
===========================================
  Files            6        8       +2     
  Lines          930     1025      +95     
===========================================
+ Hits           877      944      +67     
- Misses          53       81      +28     

Flag	Coverage Δ
unittests	92.09% <68.51%> (-2.21%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmif/utils/video_document_helper.py	66.33% <66.33%> (ø)
mmif/serialize/annotation.py	92.80% <100.00%> (+<0.01%)	⬆️
mmif/utils/__init__.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes