Some code cleanup (#22949)

* kil meh

* Datums can hold gases. e.g. pipelines

code/modules/atmospherics/gasmixtures/gas_mixture.dm

@@ -38,7 +38,7 @@ var/list/gaslist_cache = null
 	var/volume //liters
 	var/last_share
 	var/tmp/fuel_burnt
-	var/atom/holder
+	var/datum/holder
 
 /datum/gas_mixture/New(volume = CELL_VOLUME)
 	..()

code/modules/atmospherics/machinery/other/meter.dm

@@ -3,7 +3,7 @@
 	desc = "It measures something."
 	icon = 'icons/obj/meter.dmi'
 	icon_state = "meterX"
-	var/obj/machinery/atmospherics/pipe/target = null
+	var/atom/target = null
 	anchored = 1
 	power_channel = ENVIRON
 	var/frequency = 0