Write SLP files

[talmo]

This PR implements functionality to save Labels to .slp files compatible with core SLEAP.

This implementation aims to create .slp files that are nearly* identical to those produced in SLEAP v1.3.1 and FORMAT_ID = 1.2 for compatibility.

Eventually, this library will replace all of the IO modules in core SLEAP and will implement more flexible serialization formats (with legacy support).

*Because we don't load all of the same metadata as stored in SLEAP legacy files, we can't save it with byte-level equivalence. This is mostly stuff that doesn't matter or isn't used. Currently this includes:

• Internal order of nodes and edges in the underlying networkx graph (order in Skeleton.nodes and Skeleton.edges are preserved)
• Node weights (not used in legacy SLEAP anyway)
• Track.spawned_on attribute
• Suggestions
• Video backends not yet supported in sleap-io

Closes: #30

Note that this currently doesn't handle .pkg.slp files or saving embedded images just yet, but we'll handle that in the next PR.

[codecov]

Codecov Report

Merging #55 (e6a7e06) into main (ae61fb9) will increase coverage by 0.34%.
The diff coverage is 97.76%.

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
+ Coverage   94.43%   94.77%   +0.34%     
==========================================
  Files          12       12              
  Lines        1060     1187     +127     
==========================================
+ Hits         1001     1125     +124     
- Misses         59       62       +3     

Impacted Files	Coverage Δ
sleap_io/__init__.py	100.00% <ø> (ø)
sleap_io/io/main.py	94.73% <50.00%> (-5.27%)	⬇️
sleap_io/io/slp.py	97.70% <98.14%> (+0.36%)	⬆️
sleap_io/io/utils.py	100.00% <100.00%> (ø)
sleap_io/io/video.py	83.55% <100.00%> (+0.37%)	⬆️
sleap_io/model/skeleton.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more