tornado: add logic for when tornado runs out of steam

This is code from the original C/C++ release, modified to fix
what I perceived as a bug. After the tornado runs for 200 ticks,
there is a 0.2% chance of termination each tick.

git-svn-id: http://micropolis.googlecode.com/svn/trunk@912 d9718cc8-9f43-0410-858b-315f434eb58c