Further atmos changes

mixers and filters no longer have density = 1, so mobs can walk over
them.
new item: pipe freezer, can temporarily freeze pipes to allow
unwrenching when over the threshold internal pressure
note: has a temporary skin of hand labeler, needs a new sprite really.

code/ATMOSPHERICS/components/trinary_devices/filter.dm

@@ -1,7 +1,7 @@
 obj/machinery/atmospherics/trinary/filter
 	icon = 'icons/obj/atmospherics/filter.dmi'
 	icon_state = "intact_off"
-	density = 1
+	density = 0
 
 	name = "Gas filter"
 

code/ATMOSPHERICS/components/trinary_devices/mixer.dm

@@ -1,7 +1,7 @@
 obj/machinery/atmospherics/trinary/mixer
 	icon = 'icons/obj/atmospherics/mixer.dmi'
 	icon_state = "intact_off"
-	density = 1
+	density = 0
 
 	name = "Gas mixer"