Sequence returned <class 'aaf2.components.Sequence'> not implemented

## Feature Request

- [x] New Feature
- [ ] Change Existing Behavior

## Description

This sequence from Avid MCP seems to contain something that the aaf parser currently is not prepared for:
[SEQUENCE 2.zip](https://github.com/PixarAnimationStudios/OpenTimelineIO/files/5305417/SEQUENCE.2.zip)

## Context
To reproduce the issue, just try to open the sequence in otioview, it should fail after some seconds.
otioview "E:\Downloads\COMPLEX SEQUENCE 2.aaf"

```
File "c:\users\emcodem\appdata\local\programs\python\python38\lib\site-packages\opentimelineio_contrib\adapters\advanced_authoring_format.py", line 130, in _find_timecode_mobs
File "c:\users\emcodem\appdata\local\programs\python\python38\lib\site-packages\aaf2\components.py", line 204, in walk
NotImplementedError: Sequence returned not implemented
```