Adds Farm PoI

Also fixes various bugs with surface tiles, hopefully.
Adds some documentation to the map file system.
Adds a new 'plane' map, ideal for testing PoIs by loading them manually.
Makes space heaters glow orange.
Adds a new grille type for fancy surface windows.
Adds self planting hydro trays/dirts if a seed is on top of them when the map is loaded.

code/modules/maps/tg/map_template.dm

@@ -24,7 +24,8 @@ var/list/global/map_templates = list()
 	if(path)
 		mappath = path
 	if(mappath)
-		preload_size(mappath)
+		spawn(1)
+			preload_size(mappath)
 	if(rename)
 		name = rename