void on_heat( int temperature )

Called when heat is applied to an object. The 'temperature' is the amount of
heat in degrees celcius. Note that this event is used for both heating and
cooling an object (i.e. on_heat(-5) should make water freeze).

This allows you to implement effects like water freezing, bread cooking,
pottery firing, and metal burning in a furnace. Objects that apply heat
should probably do so about every 30 seconds, so timing should be based
around that idea.

It's ok for this function to destruct the object being heated.
