Added bullet detection to virus

Also:
fixed turrets not shooting from both barrels
---removed muzzlePositions array. Bullets now fire from the position of
each child object. One less step when making new turret types

---Turrets now target the first virus that enters their range and does
not target a new one until it leaves that range
------Old version was continually targeting each virus that entered the
range and resetting nextFireTime, resulting in it letting a whole swarm
through.