Add "geographical zombie density" option to enable old population method.


What it says on the tin. If enabled, zombie population density is
determined by distance from city center and city size as it was prior to
the change that calculates based on all buildings of the same type adding
a set amount to a local total. The new method eliminates vacant suburbs
entirely, whereas the old appears more random.

Both methods are pretty inflexible however. Tweaking oter_id.mondensity && MON_RADIUS will not change the fact that spawns are now chained to building type in a predictable fashion, and the back-as-an-option radial method's randomness is an illusion. I may experiment with making a third option that combines the two somehow; for buildings outside the geo radius with a monradius > 0, let rng decide