Vary cable insulation type with tier

LV cables are now paper-insulated, rather than uninsulated (which made
no sense).  MV cables are rubber-insulated as before.  HV cables are now
plastic-insulated (which they already visually appeared to be).  MV and
HV cables are still crafted by adding insulation onto lower-tier cable,
rather than by insulating raw copper; this matches the way machines are
upgraded between tiers rather than crafted afresh.

technic/machines/HV/cables.lua

@@ -2,9 +2,9 @@
 minetest.register_craft({
 	output = 'technic:hv_cable0 3',
 	recipe = {
-		{'technic:rubber',    'technic:rubber',    'technic:rubber'},
+		{'homedecor:plastic_sheeting', 'homedecor:plastic_sheeting', 'homedecor:plastic_sheeting'},
 		{'technic:mv_cable0',          'technic:mv_cable0',          'technic:mv_cable0'},
-		{'technic:rubber',    'technic:rubber',    'technic:rubber'},
+		{'homedecor:plastic_sheeting', 'homedecor:plastic_sheeting', 'homedecor:plastic_sheeting'},
 	}
 }) 
 

technic/machines/LV/cables.lua

@@ -4,7 +4,9 @@ minetest.register_alias("lv_cable", "technic:lv_cable0")
 minetest.register_craft({
 	output = 'technic:lv_cable0 6',
 	recipe = {
+		{'default:paper',        'default:paper',        'default:paper'},
 		{'default:copper_ingot', 'default:copper_ingot', 'default:copper_ingot'},
+		{'default:paper',        'default:paper',        'default:paper'},
 	}
 })