AAF Writer: Fixes the crash seen when running otio -> aaf, in some AAF files due to nesting

[freesonluxo]

This makes sure that all nested groups are in stacks, then proceeds to do the nesting correctly. Roughly

otio.schema.Stack = AAF Operation Group
otio.schema.Track = AAF Sequence

[jminor]

How does the AAF NestedScope object relate to this? I would have expected to see it in there somewhere.

[codecov-io]

Codecov Report

Merging #493 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #493      +/-   ##
==========================================
+ Coverage    88.3%   88.37%   +0.06%     
==========================================
  Files          68       68              
  Lines        6946     6969      +23     
==========================================
+ Hits         6134     6159      +25     
+ Misses        812      810       -2

Impacted Files	Coverage Δ
...neio_contrib/adapters/advanced_authoring_format.py	93.41% <100%> (+0.01%)	⬆️
...elineio_contrib/adapters/tests/test_aaf_adapter.py	98.41% <100%> (ø)	⬆️
...elineio_contrib/adapters/aaf_adapter/aaf_writer.py	95.98% <100%> (+0.72%)	⬆️

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