AAF read adapter should look for type Clip when it's finding a Marker's parent element from the Track

In merge request https://github.com/PixarAnimationStudios/OpenTimelineIO/pull/1239, it left markers with the Track if `target_track.child_at_time(marker.marked_range.start_time)` returns something that does not have `markers` property, for example `Transition`.

This is less-than ideal. Instead, it should probably look for `Clip` and attach the marker to the clip found.

Start in the AAF read adapter and compare the marker-parent lookup with pull request #1239, focusing on target_track.child_at_time(marker.marked_range.start_time). Reproduce a marker whose result is a Transition; done means the marker attaches to the containing Clip instead of remaining on the Track.

Written by the indexing model from the issue text.