Improved artillery AI

[upyr1]

Right now the only fire command that seems to work is fire at point. 

The first change is to fix the attack group and add forward observers. 

Whenever we select the attack group command if their is a friendly unit within visual range capable of acting as a forward observer the artillery should fire until the target is destroyed or the battery is out of ammo. If there is no forward observer we should get a warning like no forward observer or line of site. 

The second change would be an "on call" flag. This would add a unit to a fire support pool. Whenever a unit capable of acting as a forward observer is in line of sight of the enemy DCS will automatically start assigning fire missions to units in the fire support pool.