Ensure data format compatibility

[roomrys]

Description

In #1122, we added two new attributes in the Skeleton class called description and preview_image. Consequently, while you are able to open older files on v1.3.0a0, files from this version are unable to be opened on previous versions - which is not a problem if working on a single SLEAP version. However, this could easily become extremely annoying if working on multiple workstations which all have different versions of SLEAP installed - not to mention possibly breaking data format backwards compatibility.

The fields we added are currently not used except for the already provided template skeletons. Hence, this PR selectively writes the new fields depending on whether the skeleton is a template (manually specified via API - only by those looking to add to our collection of template skeletons). This should not break compatibility since template skeletons are only provided as early as 1.3.0a0.

Types of changes

• Bugfix
• New feature
• Refactor / Code style update (no logical changes)
• Build / CI changes
• Documentation Update
• Other (explain)

Does this address any currently open issues?

• Backward compatible (1.3.0a0 > 1.2.2 to 1.2.6) #1217

Outside contributors checklist

• Review the guidelines for contributing to this repository
• Read and sign the CLA and add yourself to the authors list
• Make sure you are making a pull request against the develop branch (not main). Also you should start your branch off develop
• Add tests that prove your fix is effective or that your feature works
• Add necessary documentation (if appropriate)

Thank you for contributing to SLEAP!

❤️

[codecov]

Codecov Report

Merging #1222 (df6cb5d) into develop (5b3b598) will decrease coverage by 0.23%.
The diff coverage is 67.24%.

@@             Coverage Diff             @@
##           develop    #1222      +/-   ##
===========================================
- Coverage    72.36%   72.14%   -0.23%     
===========================================
  Files          132      132              
  Lines        23502    23538      +36     
===========================================
- Hits         17008    16982      -26     
- Misses        6494     6556      +62     

Impacted Files	Coverage Δ
sleap/gui/learning/scopedkeydict.py	90.32% <ø> (ø)
sleap/gui/learning/dialog.py	64.42% <41.93%> (-1.45%)	⬇️
sleap/skeleton.py	91.16% <94.73%> (+0.01%)	⬆️
sleap/gui/commands.py	60.85% <100.00%> (ø)
sleap/gui/learning/receptivefield.py	85.85% <100.00%> (-0.15%)	⬇️

... and 1 file with indirect coverage changes

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[talmo]

lgtm -- probably should've made older skeletons less brittle, but this is the next best thing i guess