Ported Basic Overmap Functionality

- Ports the overmap, ships, sectors, and "landable" ships from baystation.
- Ports necessary computers to control ships and overmap shuttles.
- Shims missing machine and computer functionality pending future enhancements.
- Includes required new sprites and sounds.

code/modules/xgm/xgm_gas_mixture.dm

@@ -277,6 +277,12 @@
 
 	return removed
 
+//Returns the amount of gas that has the given flag, in moles
+/datum/gas_mixture/proc/get_by_flag(flag)
+	. = 0
+	for(var/g in gas)
+		if(gas_data.flags[g] & flag)
+			. += gas[g]
 
 //Copies gas and temperature from another gas_mixture.
 /datum/gas_mixture/proc/copy_from(const/datum/gas_mixture/sample)
@@ -478,3 +484,7 @@
 			gasmix.multiply(gasmix.volume)
 
 	return 1
+
+/datum/gas_mixture/proc/get_mass()
+	for(var/g in gas)
+		. += gas[g] * gas_data.molar_mass[g] * group_multiplier