improve attack formations, ignore feat w/o metal

- use attacker range to create semi-circle around target at close range
- ignore non-metal features (decent guess: contains the strings "heap",
"wreck" or "rock")
- make attackers move rather than move and fire (move and fire is too
easy to distract with an errant enemy unit or two)
- smarten squad congregation distance using xsize, zsize, and dist =
sqrt(pi * area), area = (xsize * zsize * 16 * numberof)