Switch cables and pipes to use define constants instead of literals for layers.

Also fix a few things to be on the correct plane.  Basically the under-floor stuff is supposed to be on PLATING_PLANE.
Portable atmospherics go on object layer so they are above stationary machinery, given that they move around.
Fixed wires to be on top of pipes like they used to be, otherwise they'd be obscured too often.

code/modules/multiz/pipes.dm

@@ -201,7 +201,7 @@ obj/machinery/atmospherics/pipe/zpipe/up/scrubbers
 	desc = "A scrubbers pipe segment to connect upwards."
 	connect_types = CONNECT_TYPE_SCRUBBER
 	piping_layer = PIPING_LAYER_SCRUBBER
-	layer = 2.38
+	layer = PIPES_SCRUBBER_LAYER
 	icon_connect_type = "-scrubbers"
 	color = PIPE_COLOR_RED
 
@@ -211,7 +211,7 @@ obj/machinery/atmospherics/pipe/zpipe/up/supply
 	desc = "A supply pipe segment to connect upwards."
 	connect_types = CONNECT_TYPE_SUPPLY
 	piping_layer = PIPING_LAYER_SUPPLY
-	layer = 2.39
+	layer = PIPES_SUPPLY_LAYER
 	icon_connect_type = "-supply"
 	color = PIPE_COLOR_BLUE
 
@@ -221,7 +221,7 @@ obj/machinery/atmospherics/pipe/zpipe/down/scrubbers
 	desc = "A scrubbers pipe segment to connect downwards."
 	connect_types = CONNECT_TYPE_SCRUBBER
 	piping_layer = PIPING_LAYER_SCRUBBER
-	layer = 2.38
+	layer = PIPES_SCRUBBER_LAYER
 	icon_connect_type = "-scrubbers"
 	color = PIPE_COLOR_RED
 
@@ -231,6 +231,6 @@ obj/machinery/atmospherics/pipe/zpipe/down/supply
 	desc = "A supply pipe segment to connect downwards."
 	connect_types = CONNECT_TYPE_SUPPLY
 	piping_layer = PIPING_LAYER_SUPPLY
-	layer = 2.39
+	layer = PIPES_SUPPLY_LAYER
 	icon_connect_type = "-supply"
 	color = PIPE_COLOR_BLUE

code/modules/power/cable.dm

@@ -53,7 +53,8 @@ var/list/possible_cable_coil_colours = list(
 	icon_state = "0-1"
 	var/d1 = 0
 	var/d2 = 1
-	layer = 2.44 //Just below unary stuff, which is at 2.45 and above pipes, which are at 2.4
+	plane = PLATING_PLANE
+	layer = WIRES_LAYER
 	color = COLOR_RED
 	var/obj/machinery/power/breakerbox/breaker_box
 

code/modules/power/cable_ender.dm

@@ -8,7 +8,8 @@
 	icon = 'icons/obj/power_cond_heavy.dmi'
 	name = "large power cable"
 	desc = "This cable is tough. It cannot be cut with simple hand tools."
-	layer = 2.39 //Just below pipes, which are at 2.4
+	plane = PLATING_PLANE
+	layer = PIPES_LAYER - 0.05 //Just below pipes
 	color = null
 	unacidable = 1
 	var/id = null

code/modules/power/cable_heavyduty.dm

@@ -7,7 +7,8 @@
 	icon = 'icons/obj/power_cond_heavy.dmi'
 	name = "large power cable"
 	desc = "This cable is tough. It cannot be cut with simple hand tools."
-	layer = 2.39 //Just below pipes, which are at 2.4
+	plane = PLATING_PLANE
+	layer = PIPES_LAYER - 0.05 //Just below pipes
 	color = null
 
 /obj/structure/cable/heavyduty/attackby(obj/item/W, mob/user)

code/modules/recycling/disposal.dm

@@ -663,6 +663,7 @@
 	var/dpdir = 0		// bitmask of pipe directions
 	dir = 0				// dir will contain dominant direction for junction pipes
 	var/health = 10 	// health points 0-10
+	plane = PLATING_PLANE
 	layer = DISPOSAL_LAYER	// slightly lower than wires and other pipes
 	var/base_icon_state	// initial icon state on map
 	var/sortType = ""