Add "spillover" behavior

Change the behavior of the part where the population gets thrown on the map so that we can have a limit on the number of people per cell
Approach: Throw population on map as we do now, but add a step where we reduce the population in the cells that are above the limit back to the limit, and move the extra people randomly to the adjacent 9 cells (only those that are in the same country, otherwise our numbers won't match the UN numbers!)
Later: Allow this limit to change over time (to follow the lifecycle of cities, where densities tend to go down at a certain point).

crstn / CISC