Ice labs and get_temperature() function

Half the labs will now be "ice labs", which are labs that get increasingly cold as you descend the depths.

To accomplish this, the get_temperature function was added. It checks to see if the player is in an ice lab, and if so it returns a different temperature value. The weather's temperature value is not changed, so weather will not be affected. It would be trivial to add temperature variation for more buildings.

I have tested this a little bit. I've walked around in both kinds of labs to make sure there is no sillyness. A few flaws in body temperature were made obvious, such as frostbite not doing anything.