some fixes regarding gravity and cyberdiscs.

makes cyberdiscs explode on the level they die.
makes dead flying units fall.
make large corpse fall properly, and clear the tiles properly when they do.
move the instakill() call to the unitDieBState instead of the point of explosion.
adds a pointer to the battlegame in battlescapeState
that doesn't go there.

whoops, made a reminder to myself of what class i needed access to, and
did it in code rather than comment :S
some more fixes regarding cyberdiscs

that looks better.
is there anything RIGHT in the gravity code?
play death sounds at the right time.

when they start FALLING, not when they start turning.
that looks like some debug code

or it's not, either way, that DEFINITELY shouldn't be there.
store the alien rank as a numerical value

... you know in case we might need it.
wat
patrol code based on Ufopaedia data

see http://www.ufopaedia.org/index.php?title=ROUTES.DAT
"flags" now used to direct aliens other than scouts
seems to work great in UFOs but not in Final Assault (commanders roam whole
base)
don't make everyone scouts in base assaults!
better patrol behavior

aliens prefer to move to nodes with their equivalent rank, like xcom1
aliens prefer to move when standing in fire
aliens will look for higher value nodes when assaulting xcom base, like xcom1