Writer: Added pre- / post-transcribe hooks to facilitate media essence embedding

[timlehr]

Link the Issue(s) this Pull Request is related to.

Requires #49

Requires change to OTIO core (for tests to pass):
AcademySoftwareFoundation/OpenTimelineIO#1711

Summarize your change.

This PR adds AAF adapter specific hooks that can be used to implement just-in-time transcoding of media for embedding purposes. It also adds the embed_essence argument to the write functionality, which will attempt to import media references. The new create_edgecode argument allows to control if an edge code should be applied to the AAF. Only .wav, .dnx and .aaf can be embedded. Thanks a lot @markreidvfx for your help with the embedding code.

New hooks introduced:

• otio_aaf_pre_write_transcribe: useful for transcoding media for embedding
• otio_aaf_post_write_transcribe: useful for cleaning up in-memory timeline / reverting to original media
• otio_aaf_pre_read_transcribe: useful to modify AAF structure in-memory before read
• otio_aaf_post_read_transcribe: useful to extract media from AAF file post timeline transcription

Reference associated tests.

Draft PR since I'm missing a test for .aaf embedding still

tests/test_aaf_adapter.py

[timlehr]

@markreidvfx I confirmed all but one of the tests producing valid results in Avid MC. However I can't get the audio import test test_transcribe_embed_wav_audio to produce results that MC can load. It errors out on import. Any chance you could have a look at this? Otherwise this PR is good for review now. :)

[timlehr]

@reinecke @jminor After our discussion I reworked the code a little and it now offers both read and write hooks to facilitate embedding & extracting of media. I also added test coverage for all supported media types using snippets from Picchu.