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)
Partial-tile firedoors now default to full-tile firedoors as the part-tile ones were breaking ZAS zones entirely.  Possible cause of #2627 and maybe #2451, but those need tested and I did not have time.
Merge pull request #2644 from SkyMarshal/master

ZAS fixes: #2632 #2630 #1865