Several ZAS fixes (Fire, firedoors, flamethrowers, and grenades oh my!)

#2632: firedoors respected, flamethrower uses liquid fuel subtype, flame grenades now make a puddle of welder fuel that is lit aflame with napalm
#2630: The code has been calling (turf).temperature_expose when it was supposed to be hotspot_expose
#1865: (aqain) Added a check to remove trace gas fuel under a certain point.

Also: added a flashpoint value for plasma, which is the temperature at which it self-ignites (Currently 246C or 519K which is equal to gasoline)