Adds gas mixing proc and updates gas mixer

Also includes a whole bunch of fixes and updates for atmos machinery

code/ATMOSPHERICS/components/omni_devices/filter.dm

@@ -8,6 +8,8 @@
 	var/list/filters = new()
 	var/datum/omni_port/input
 	var/datum/omni_port/output
+	
+	var/target_pressure = ONE_ATMOSPHERE
 
 /obj/machinery/atmospherics/omni/filter/Del()
 	input = null

code/ATMOSPHERICS/components/omni_devices/mixer.dm

@@ -13,6 +13,8 @@
 	var/tag_south_con
 	var/tag_east_con
 	var/tag_west_con
+	
+	var/target_pressure = ONE_ATMOSPHERE
 
 /obj/machinery/atmospherics/omni/mixer/New()
 	..()

code/ATMOSPHERICS/components/omni_devices/omni_base.dm

@@ -11,7 +11,7 @@
 
 	var/on = 0
 	var/configuring = 0
-	var/target_pressure = ONE_ATMOSPHERE
+	//var/target_pressure = ONE_ATMOSPHERE	//a base type as abstract as this should NOT be making these kinds of assumptions
 
 	var/tag_north = ATM_NONE
 	var/tag_south = ATM_NONE