otio.algorithms.flatten_stack() only flattens one level

[jminor]

If you have Tracks or Stacks within a Track, otio.algorithms.flatten_stack() doesn't flatten all the way.

We might need a separate otio.algorithms.flatten_track() and a recursive flatten() that deals with arbitrary nesting.

[meshula]

What's the current state of flattening @jminor, is there work remaining on this?