add ability to harvest units

[andy5995]

Right now, only resources can be harvested, but not units.

If this were implemented, animals could be harvested

ZetaGlest/zetaglest-data#26

And we'd have more options to implement traders or a way to generate resources

ZetaGlest/zetaglest-data#29

[andy5995]

Wow! Thanks, @mathusummut

Could you demo this by making Persian sheep harvestable? Basically having sheep wouldn't provide food, until they are harvested, is the way I see it.