Optionally use the I3s height coordinates

Please make it optional to import and place slpk buildings on the right height on the surface. Now, buildings are always placed on the surface, even if the placement of the building in the slpk file differs from that. For example a basement is also placed on the surface, even though that is an underground building like a basement.

Documentation:

https://previewsupport.tygron.com/wiki/How_to_import_a_SLKP_file