Submap temps

Two new functions, map::set_temperature and map::temperature. The first sets the submap's temperature, and the second fetches it.

Ice labs now have their own submap temperature, meaning in the future it can be changed.

There is one elusive issue : despite the code saying "int temperature = -20 + 30*(g->levz);", when the player goes from ground level to one level lower, the temperature modifier stays are -20. I can't even begin to understand why that would be.