[request] Adding TradeYieldModifier to buildings

[Panjandrum]

I have been trying to add the TradeYieldModifier tag used in civics and traits to a building, but have found it much more difficult than I expected.

It seemed a simple enough modification, but my attempts to cut and paste the relevent lines from cvTraitInfos to cvBuildingInfos in CvInfos.cpp and CvInfos.h resulted in a dll that wouldnt compile.

If anyone could compile a dll with this property, I will reward you with absolution for your sins.