refactor: move traffic-density mutation code to new function
cleanup: use getTrafficDensity() instead of directly accessing the array
refactor: make public getLandValue() method for accessing landValueMem[]
overlay: use public getLandValue() method to draw landvalue overlay

instead of directly accessing the landValueMem[] array
city-eval: use public getTrafficDensity() instead of direct access

of trfDensity[] array