Finished incorporating turretEffectiveRange into turret targeting. Now the turret picks the closest targetable actor to target and attempts to kill it.
Removed unused methods from collisionHandler.
There is an issue with the turret shooting twice; I think it happens because the updates occur in this order of events:
 (object creations)
turret creates beam
(interactive collisions)
effectiveRange hits actor, adding its shape to the turret's availableTargets;
beam hits actor, giving it die command;
(garbage collection)
level sees actor is dying, so it removes it from its lists, but its shape is still stored in the turrets available targets