Nerfing guard towers & other balance changes

- Range of obelisk, Adv. Guard Tower and SAM site reduced.
- Damage of Guard Tower reduced vs. infantry and light vehicles; spread increased to compensate a bit.
- Damage of Advanced Guard Tower nerfed, especially against light vehicles. Damage vs. infantry increased (though it's still low DPS). Spread also increased to compensate for its lack of power vs. infantry.
- Turret damage mostly unchanged, but does less damage per shot. Damage vs. light vehicles increased to match its damage vs. heavy armor. Damage vs. infantry slightly reduced.