Added ClonesProductionUnits trait for RA2-style Cloning Vats

Yep we're in a feature freeze.
Yep we don't have RA2 yet.

I think mod authors will be happy to have this a few games early.
This theoretical `INotifyOtherProduced` interface will also be useful for HackyAI (it can listen for production instead of polling the world for new units).