OTIO edit commands

[ggarra13]

Feature Request

This is a feature request to add editing commands based on #711 and experimental PR #1518.

Description

I am currently working on adding all the commands described in #711, without Undo/Redo capabilities and, as a first step, without taking into account transitions. I am currently adding them to a new namespace (edit), instead of algo as it was discussed.

Context

I currently have (with unit tests): slice, overwrite, insert, slip, slide and (without checking available_range) trim.
I am still missing: finishing trim, and work on ripple, roll and fill.