Fixes gas mixture bugs, other bugs, and cleanup

In particular, gas_mixture/remove() was not accounting for
group_multiplier when determining the maximum amount of gas to remove
Also cleans up the interface for gas_mixture/heat_capacity().

code/ATMOSPHERICS/components/unary/outlet_injector.dm

@@ -54,7 +54,7 @@
 	..()
 	injecting = 0
 
-	if(!on || stat & NOPOWER)
+	if(!on)	//only uses power when injecting
 		return 0
 	
 	var/datum/gas_mixture/environment = loc.return_air()

code/ATMOSPHERICS/components/unary/vent_scrubber.dm

@@ -256,6 +256,8 @@
 	..()
 	if (get_dist(usr, src) <= 1)
 		usr << "A small gauge in the corner reads [round(last_flow_rate, 0.1)] L/s; [round(last_power_draw)] W"
+	else
+		usr << "You are too far away to read the gauge."
 
 /obj/machinery/atmospherics/unary/vent_scrubber/Del()
 	if(initial_loc)