Refactors gravity checking (#4591)

Removes a lot of duplicated or redundant code from areas when checking gravity.

code/modules/mob/animations.dm

@@ -90,7 +90,7 @@ note dizziness decrements automatically in the mob's Life() proc.
 	var/turf/turf = get_turf(src)
 	if(!istype(turf,/turf/space))
 		var/area/A = turf.loc
-		if(istype(A) && A.has_gravity)
+		if(istype(A) && A.has_gravity())
 			make_floating(0)
 			return
 		else if (Check_Shoegrip())

code/modules/mob/inventory.dm

@@ -327,7 +327,7 @@ var/list/slot_equipment_priority = list( \
 
 		if(!src.lastarea)
 			src.lastarea = get_area(src.loc)
-		if((istype(src.loc, /turf/space)) || (src.lastarea.has_gravity == 0))
+		if((istype(src.loc, /turf/space)) || (src.lastarea.has_gravity() == 0))
 			src.inertia_dir = get_dir(target, src)
 			step(src, inertia_dir)
 

code/modules/mob/living/living.dm

@@ -576,7 +576,7 @@ default behaviour is:
 
 						if(!istype(M.loc, /turf/space))
 							var/area/A = get_area(M)
-							if(A.has_gravity)
+							if(A.has_gravity())
 								//this is the gay blood on floor shit -- Added back -- Skie
 								if (M.lying && (prob(M.getBruteLoss() / 6)))
 									var/turf/location = M.loc

code/modules/mob/mob_movement.dm

@@ -436,7 +436,7 @@
 
 	if(!lastarea)
 		lastarea = get_area(loc)
-	if(!lastarea.has_gravity)
+	if(!lastarea.has_gravity())
 		return 0
 
 	return 1
@@ -451,7 +451,7 @@
 			return 1
 		else
 			var/area/A = T.loc
-			if(A.has_gravity || shoegrip)
+			if(A.has_gravity() || shoegrip)
 				return 1
 
 	for(var/obj/O in orange(1, src))