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.
No. I didn't use the word `Queue` here because there is no queuing happening.
When a valid unit type is trained this trait causes `self` to immediately produce a clone of the original unit. I will add a trait `Desc[()]` to clarify; good idea.