-tg- SpaceMove update

This commit updates the spacemove system to -tg-'s; All objects now drift
in space, not just mobs. A few hardcoded space checks have been replaced
with has_gravity(); So it applies to zero-G as well as space.

Spacepods no longer drift, because their snowflakey shitcode just doesn't
work. So they just move.

code/modules/power/singularity/singularity.dm

@@ -268,6 +268,8 @@
 
 	step(src, movement_dir)
 
+/obj/singularity/Process_Spacemove() //The singularity stops drifting for no man!
+	return 0
 
 /obj/singularity/proc/check_turfs_in(var/direction = 0, var/step = 0)
 	if(!direction)