Reduced starting zombie count and changed zombie behavior to fix end game stalling behavior.

I changed the starting zombie human ratio to 40/10. I also made zombies check to see if the nearest human is less than six times further than the nearest zombie to determine zombie spreading behavior. This makes the zombies cluster when hunting and fixes some biting issues but allows the zombies to spread out the swarm a tiny bit in the early/ mid game and it avoids clustering in the late game when humans are scarce.