Adds a setter for density (#59529)

Adds set_density()
Fixes one instance of a duplicate density assignment on an object.
Comments two hacky usages of density which will have to forgo using the setter for now.

Lets us append code to the event of density changing.
Pretty sure this is leading up to some multitile object thing -Lemon

code/modules/mining/fulton.dm

@@ -83,7 +83,7 @@ GLOBAL_LIST_EMPTY(total_extraction_beacons)
 					M.buckled.unbuckle_mob(M, TRUE) // Unbuckle them to prevent anchoring problems
 			else
 				A.set_anchored(TRUE)
-				A.density = FALSE
+				A.set_density(FALSE)
 			var/obj/effect/extraction_holder/holder_obj = new(A.loc)
 			holder_obj.appearance = A.appearance
 			A.forceMove(holder_obj)
@@ -134,7 +134,7 @@ GLOBAL_LIST_EMPTY(total_extraction_beacons)
 			sleep(4)
 			holder_obj.cut_overlay(balloon3)
 			A.set_anchored(FALSE) // An item has to be unanchored to be extracted in the first place.
-			A.density = initial(A.density)
+			A.set_density(initial(A.density))
 			animate(holder_obj, pixel_z = 0, time = 5)
 			sleep(5)
 			A.forceMove(holder_obj.loc)