tweak the Astral Plane level

tweak the Astral Plane level

Even out the difficulty (from one game to another) somewhat.  Instead
of a 75% chance that two large areas will be opened up on the left
and right sides of the arrival area plus 13.5 (avg) * 1.5 (avg) extra
monsters in that region, change to 60% chance that the left side will
be opened up with 7 (avg) * 1.5 (avg) extra monsters and a separate
60% chance that the right side will be opened up with 7 * 1.5 (avg)
extra monsters.  The chance that both sides get opened up drops to
36% but the chance that neither side gets opened drops to 16%, with
difference made up by 24% chance each for just one side or the other.

I was a little surprised that this actually worked.  I hope there's
a less clumsy way to have a loop index.