Found more than one Source Clip (4) for sourceClipComposition case. This is unexpected

I am trying to use OTIO to rearrange some clips in an AVID timeline by exporting the timeline as an AAF and manipulate it in python. I can interact with the clips but I'm getting an error when they are read in and my output AAF's masterclip (it should be a subclip) is too short.

Are there known issues with subclips?

When I run the code, I'm getting this error:
`Found more than one Source Clip (4) for sourceClipComposition case. This is unexpected`

## To Reproduce

1. Operating System: macOS 10.14.6
2. Python version 3.8 and 3.9
3. Example snippet that demonstrates the issue
```
import opentimelineio as otio

timeline = otio.adapters.read_from_file("VINYL.aaf")
for clip in timeline.each_clip():
print (clip.name)

anything = otio.adapters.write_to_file(timeline, "mydemo.aaf")

print (anything)
```

4. OpenTimelineIO release version: OpenTimelineIO 0.13.0
5. Compiler information:
Apple clang version 11.0.0 (clang-1100.0.33.17)
```
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
```

## Expected Behavior
I'd imagine I should just be able to see the clips as subclips. I assumed OTIO supported subclips as I saw it in the documentation.

Start by reproducing the report with the Python snippet, VINYL.aaf, and the adapters.read_from_file and write_to_file entry points. Investigate how the AAF input's sourceClipComposition is interpreted, then verify that reading and writing preserves the expected subclips and duration without the multiple-Source-Clip error.

Written by the indexing model from the issue text.