tweaks to balance: artillery min. range & more

Artillery was recently buffed, but I think this makes the fact that they can currently fire at close range is not appropriate. I think artillery should be vulnerable to units that are able to get quite close, instead of being able to fire at them and do massive damage.

The reason min. range of 2c896 was picked is that when you set min. range to 3c0, it is not able to fire exactly 3 cells away. Perhaps the behaviour should change so that 3c0 = 3 cells away. 

(For comparison, MLRS has minimum range of 3).