Pipe network datum Destroy() cleanup

* Every pipe in a pipeline has a reference to the pipeline.  This needs to be cleared, both so the pipeline can be gc'd, and also so the pipes don't try and keep using the qdel'd pipeline.
* Same story for each pipeline in a pipe network, and each machine in a pipe network.
* Pipe networks are also in the pipe_networks global list.  While the controller would detect and remove it from the list on the next tick, cleaning up ourselves is the responsible thing to do.

code/ATMOSPHERICS/datum_pipe_network.dm

@@ -16,6 +16,15 @@ datum/pipe_network
 
 		..()
 
+	Destroy()
+		pipe_networks -= src
+		for(var/datum/pipeline/line_member in line_members)
+			line_member.network = null
+		for(var/obj/machinery/atmospherics/normal_member in normal_members)
+			normal_member.reassign_network(src, null)
+		gases.Cut()  // Do not qdel the gases, we don't own them
+		return ..()
+
 	proc/process()
 		//Equalize gases amongst pipe if called for
 		if(update)

code/ATMOSPHERICS/datum_pipeline.dm

@@ -14,6 +14,8 @@ datum/pipeline
 
 		if(air && air.volume)
 			temporarily_store_air()
+		for(var/obj/machinery/atmospherics/pipe/P in members)
+			P.parent = null
 
 		. = ..()