Fix supply converter

Its registration as a battery (BA) node didn't work.  It needs to be
registered as both a producer (PR) and a receiver (RE).

technic/machines/supply_converter.lua

@@ -76,6 +76,6 @@ minetest.register_abm({
 })
 
 for tier, machines in pairs(technic.machines) do
-	technic.register_machine(tier, "technic:supply_converter", technic.battery)
+	technic.register_machine(tier, "technic:supply_converter", technic.producer_receiver)
 end
 

technic/machines/switching_station.lua

@@ -106,6 +106,9 @@ local check_node_subp = function(PR_nodes, RE_nodes, BA_nodes, all_nodes, pos, m
 			add_new_cable_node(PR_nodes, pos)
 		elseif machines[name] == technic.receiver then
 			add_new_cable_node(RE_nodes, pos)
+		elseif machines[name] == technic.producer_receiver then
+			add_new_cable_node(PR_nodes, pos)
+			add_new_cable_node(RE_nodes, pos)
 		elseif machines[name] == technic.battery then
 			add_new_cable_node(BA_nodes, pos)
 		end

technic/register.lua

@@ -3,6 +3,7 @@
 
 technic.receiver = "RE"
 technic.producer = "PR"
+technic.producer_receiver = "PR_RE"
 technic.battery  = "BA"
 
 technic.machines    = {}