Stereo (SDL) Support

OTIO should support stereo SDL metadata.

Here is some reference for how SDL information is stored in EDLs:
https://knowledge.autodesk.com/support/flame-products/learn-explore/caas/CloudHelp/cloudhelp/2017/ENU/Lustre/files/GUID-D29FB230-C09D-4894-B8CD-CF6823E96224-htm.html
http://help.autodesk.com/view/LUSTRE/2018/ENU/?guid=GUID-455BC460-B1C9-4B6A-8047-6375959B67E7
