Fixes issues: #2632 Fire Issues (Fire doors work, Flamethrower reverted to making a flamethrower 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)