AAF Reader: Sequence with nested scopes unable to be reconstructed

Now that we are able to process an AAF containing nested sequences and start analysing the resulting OTIO, I've come across this first issue, which is a pretty big one really.

The clip inside the sequence that has been exported simply has the following information:

```
"OTIO_SCHEMA": "Clip.1",
"effects": [],
"markers": [],
"media_reference": {
"OTIO_SCHEMA": "MissingReference.1",
"available_range": null,
"metadata": {},
"name": null
},
"metadata": {
"AAF": {
"ClassName": "SourceClip",
"DataDefinition": {
"Description": "Picture data",
"Identification": "01030202-0100-0000-060e-2b3404010101",
"Name": "DataDef_Picture"
},
"Length": 120,
"Name": "SourceClip Missing Mob?",
"SourceID": "urn:smpte:umid:00000000.00000000.00000000.00000000.00000000.00000000.00000000.00000000",
"SourceMobSlotID": 0,
"StartTime": 0
}
},
"name": "SourceClip Missing Mob?",
"source_range": null
```

There is clearly an issue, what with the name showing up as `"SourceClip Missing Mob?"` so worth looking in to what's happening there. Inspecting the OTIO object itself, the two nested sequences in question are present, existing as their own `Timeline` structures inside a top-level `SerializableCollection` structure, which would make it possible to parse and recreate, but without any info in the main timeline's clip, this obviously isn't able to be done.

Given the way that this nested structure is exported out to AAF, is it possible for it to be treated the same way as with a nested XML e.g. each nested scope existing as a `Stack` on a `Track` and having it's own internal hierarchy going down the levels?

Zip attached with example AAF & XML:

[SpeedChange_MstrSeq_withNesting_Examples_190408.zip](https://github.com/PixarAnimationStudios/OpenTimelineIO/files/3052794/SpeedChange_MstrSeq_withNesting_Examples_190408.zip)

Start by reproducing the issue with the attached SpeedChange_MstrSeq_withNesting_Examples_190408.zip, comparing the AAF and XML examples with the resulting OTIO SerializableCollection and nested Timelines. Trace how the AAF Reader handles the SourceClip with missing mob information; done means nested sequences are reconstructed in the main timeline with their internal hierarchy preserved.

Written by the indexing model from the issue text.