Convert USD instances to maya instances on import

[pmolodo]

Is your feature request related to a problem? Please describe.
Occasionally, we want to import a USD scene that uses instancing into maya, and be able to preserve those instances.

Describe the solution you'd like
Have a setting to allow maya to automatically convert usd-instances to maya-instances on import.

Additional context
So, we've actually already implemented this, and it works and has been used in production, though only infrequently. Was updating some of our branches recently, and figured I might as well push it public, in case someone else is interested:

https://github.com/LumaPictures/maya-usd/tree/pr/pxr/maya_importing_instances

Mostly, I think it needs tests added before I think it would be ready for a PR; wasn't sure if I should create this as a WIP PR, or an issue...