Video backends

[talmo]

This PR implements the core video backends for media files, HDF5 and embedded images.

[codecov]

Codecov Report

Merging #31 (aa7afd0) into main (997d494) will decrease coverage by 13.93%.
The diff coverage is 44.12%.

❗ Current head aa7afd0 differs from pull request most recent head 34bdc59. Consider uploading reports for the commit 34bdc59 to get more accurate results

@@             Coverage Diff             @@
##             main      #31       +/-   ##
===========================================
- Coverage   98.51%   84.59%   -13.93%     
===========================================
  Files          11       12        +1     
  Lines         741      980      +239     
===========================================
+ Hits          730      829       +99     
- Misses         11      151      +140     

Impacted Files	Coverage Δ
sleap_io/io/video.py	38.23% <38.23%> (ø)
sleap_io/model/labeled_frame.py	96.87% <66.66%> (-3.13%)	⬇️
sleap_io/model/video.py	71.87% <67.85%> (-28.13%)	⬇️
sleap_io/io/slp.py	97.82% <71.42%> (-2.18%)	⬇️
sleap_io/model/instance.py	100.00% <100.00%> (ø)
sleap_io/model/labels.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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