Add video keyframe extraction

[wonjuleee]

Summary

Depends on #100408.

• Added ZNCC-based keyframe extractor feature

How to test

datum create -o proj
datum import -f video_keyframes -p proj video.mp4 -t 0.3 --
datum export -p proj -f image_dir --

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Base: 83.92% // Head: 83.95% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (74bd629) compared to base (6f481cc).
Patch coverage: 95.12% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #791      +/-   ##
===========================================
+ Coverage    83.92%   83.95%   +0.02%     
===========================================
  Files          173      173              
  Lines        22739    22779      +40     
  Branches      4946     4951       +5     
===========================================
+ Hits         19083    19123      +40     
  Misses        2535     2535              
  Partials      1121     1121              

Flag	Coverage Δ
macos-11_Python-3.10	83.87% <95.12%> (+0.02%)	⬆️
macos-11_Python-3.7	83.55% <94.87%> (+0.02%)	⬆️
macos-11_Python-3.8	83.79% <95.12%> (+0.02%)	⬆️
macos-11_Python-3.9	83.81% <95.12%> (+0.03%)	⬆️
ubuntu-20.04_Python-3.10	83.87% <95.12%> (+0.02%)	⬆️
ubuntu-20.04_Python-3.7	83.55% <94.87%> (+0.02%)	⬆️
ubuntu-20.04_Python-3.8	83.80% <95.12%> (+0.02%)	⬆️
ubuntu-20.04_Python-3.9	83.81% <95.12%> (+0.02%)	⬆️
windows-2019_Python-3.10	83.83% <95.12%> (+0.02%)	⬆️
windows-2019_Python-3.7	83.51% <94.87%> (+0.02%)	⬆️
windows-2019_Python-3.8	83.75% <95.12%> (+0.02%)	⬆️
windows-2019_Python-3.9	83.77% <95.12%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
datumaro/plugins/data_formats/video.py	92.85% <95.12%> (+1.94%)	⬆️
datumaro/cli/contexts/project/__init__.py	72.68% <0.00%> (+0.97%)	⬆️

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[bonhunko]

LGTM!

[sooahleex]

Looks good to me. Just one minor comment, I think It seems good to add comment for adding epsilon.

[vinnamkim]

LGTM.