Houses on hillsides

- Houses can now be placed on hillsides and the models will adapt their height to the steep terrain.
- This is only available for housing zoning, not for worskphops, farms, or market zoning.
- Single trees and columns can also be built on steep terrain.
New map: "Lugalbanda"

- New map with expert difficulty : lower yield from fields and less farmable area, for players who want a greater challenge.
Other changes:
Fixed: Flower boxes not being built.
Some minor bugs and corrections.