Transition duration incorrect in CMX 3600

[JoshBurnell]

Hey, guys,

The cmx_3600.py adapter appears to use a duration for Transitions that doesn't conform to the CMX 3600 spec.

According to this spec (Page 30), the duration is stored in a "RATE" field.

This guide makes it a little easier to read:

75 frame dissolve from reel 123 to reel 123B on video and A1:

Edit # Reel Name  Channel Trans      Dur      Source IN          Source OUT          Record IN         Record OUT
105 123    B     C        03:05:57:17 03:05:57:17 01:00:21:20 01:00:21:20
105 123B   B     D    075 03:15:33:09 03:15:35:24 01:00:21:20 01:00:24:05

However, the cmx_3600.py adapter appears to use the length of the previous clip:
https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/84c8245c070b12d7e627c145435a5db9add0d2de/src/py-opentimelineio/opentimelineio/adapters/cmx_3600.py#L668

Test EDL:
cmx_3600_transition.edl.zip

Recreation Steps:

In [13]: cat cmx_3600_transition.edl
TITLE:   0800_Cut_v3_ToLayout_021121_EDL 
FCM: NON-DROP FRAME
001  ABC0100. V     C        01:00:12:15 01:00:17:01 01:00:48:20 01:00:53:06 
002  ABC0200. V     C        01:00:11:07 01:00:14:09 01:00:53:06 01:00:56:08 
003  ABC0200. V     C        01:00:14:09 01:00:14:09 01:00:56:08 01:00:56:08 
003  ABC0300. V     D    026 01:00:10:08 01:00:13:09 01:00:56:08 01:00:59:09 
* BLEND, DISSOLVE 
004  ABC0400. V     C        01:00:11:09 01:00:17:20 01:00:59:09 01:01:05:20

In [14]: import opentimelineio as otio

In [15]: timeline = otio.adapters.read_from_file("cmx_3600_transition.edl")

In [16]: timeline.tracks[0][2].duration()
Out[16]: otio.opentime.RationalTime(value=73, rate=24)