Adjust zombie behavior

[christophercarney]

Zombies are sort of useless but they're fun to kill. They should center themselves around a nearby enemy (that's not a zombie) at a certain R radius away so you can't slash both the zombie and the enemy he's guarding, even if the enemy is in another shift zone.

This gives zombies a purpose as meat shields and also confuses the player in a good way and makes them stop and think about which zone the enemy is in if they're crowding around shifted enemies.

If there is no one left to guard they should return to their default behavior of just wandering around