Xenoarch tweaks

Created by: JMWTurner

• Drilling like a madman through an asteroid turf that contains xenoarchaeological finds is now much less likely to yield a find (down from 25% to 5%)
• Fixed an exploit that allowed people to find large artifacts way too easily by pocking asteroid turfs with a small/hand pickaxe then drilling it. Using a large pickaxe/drill will now always destroy whatever large artifact might that turf contain.
• Balanced artifact debris that spawn when breaking finds. Drilling through an excavation field will now mostly yield crap.