Add Blaze's Turret mod

per http://smf.cataclysmdda.com/index.php?topic=8060.msg190017#msg190017

This is the Vehicle Turret Mod, by Blaze. I have not done any work on this other than submitting it here, so all credit goes to Blaze.
Only seeing three groups of SMG here.

Thinking that matched pairs would make a bit more sense from the internal-infrastructural perspective (so two groups of 2 of the same SMG), but I'm not a machinist.