Fires generate hot air [CR]

As per Kevin's mention in #9016, this PR makes it so fires generate a hot air field now. It spreads like mad, and dissipates quickly. By my math, with a half-life of 80 turns, it should be able to warm up an area of 9x9 decently. I lit a little fire in the evac center and the results were that the corner was warm, but only about one quarter of the building felt the heat. The field dissipates even quicker outdoors, hopefully to the point where it's barely noticeable and that the radiated heat of the fire is what is warming you up.

Once my windchill PR is up to par, I will make this hot air field lower humidity as well.

I lit a phone book on fire in the backroom of a grocery store, and it had a hard time keeping level 3 "hot air" going. I might need to up the half-life. However, the fact that I didn't proof read my code means that hot air can generate on walls (if a fire is next to it), and then permeate outdoors. And that's okay I think!

It would be nice to have finer resolution of this field, like 10 levels instead of 3.

I've also written this thinking that heat comes and goes quickly; altering the half life and the rate of generation could give this heat more of a lingering presence.
Being "on fire" increases your convergent-temp very intensely, and is highly dangerous