AAF adapter: track timecode offsets clip source range

## Feature Request

New feature

## Description

Currently the AAF adapter adds the track timecode to the source range of any clip. This allows for roundtripping of the information, but makes it cumbersome to retrieve the "real" source range for clips. This behaviour could be made optional with a new adapter argument, so that correct source range information is more easily accessible.

## Context

Related bit of code:

https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/master/contrib/opentimelineio_contrib/adapters/advanced_authoring_format.py#L303-L305

Start with contrib/opentimelineio_contrib/adapters/advanced_authoring_format.py around lines 303-305, where the AAF adapter adds track timecode to a clip's source range. Determine how an optional adapter argument can expose the real source range while retaining roundtripping behavior; done means the behavior is selectable and the source range is accessible without the offset when requested.

Written by the indexing model from the issue text.