Introduce constants related to the net floor area of a building

The constants ROOF_STORIES_AREA_FACTOR and NET_FLOOR_AREA_FACTOR should be introduced. They have been implemented in globaldynamicexposure/rabotnik/rabotnik-rules!28 (merged), but the second one needs to be referenced in globaldynamicexposure/exposure-finalizer!68 (merged)

\fyi @all