reorganize xgm into its own module

Signed-off-by: Mloc <colmohici@gmail.com>

code/defines/gases.dm

@@ -0,0 +1,56 @@
+/decl/xgm_gas/oxygen
+	id = "oxygen"
+	name = "Oxygen"
+	specific_heat = 20	// J/(mol*K)
+	molar_mass = 0.032	// kg/mol
+
+	flags = XGM_GAS_OXIDIZER
+
+/decl/xgm_gas/nitrogen
+	id = "nitrogen"
+	name = "Nitrogen"
+	specific_heat = 20	// J/(mol*K)
+	molar_mass = 0.028	// kg/mol
+
+/decl/xgm_gas/carbon_dioxide
+	id = "carbon_dioxide"
+	name = "Carbon Dioxide"
+	specific_heat = 30	// J/(mol*K)
+	molar_mass = 0.044	// kg/mol
+
+/decl/xgm_gas/phoron
+	id = "phoron"
+	name = "Phoron"
+	specific_heat = 200	// J/(mol*K)
+	
+	//Hypothetical group 14 (same as carbon), period 8 element.
+	//Using multiplicity rule, it's atomic number is 162
+	//and following a N/Z ratio of 1.5, the molar mass of a monatomic gas is:
+	molar_mass = 0.405	// kg/mol
+
+	tile_overlay = "phoron"
+	overlay_limit = 0.7
+	flags = XGM_GAS_FUEL | XGM_GAS_CONTAMINANT
+
+/decl/xgm_gas/volatile_fuel
+	id = "volatile_fuel"
+	name = "Volatile Fuel"
+	specific_heat = 253	// J/(mol*K)	C8H18 gasoline. Isobaric, but good enough.
+	molar_mass = 0.114	// kg/mol. 		same.
+
+	flags = XGM_GAS_FUEL
+
+/decl/xgm_gas/sleeping_agent
+	id = "sleeping_agent"
+	name = "Sleeping Agent"
+	specific_heat = 40	// J/(mol*K)
+	molar_mass = 0.044	// kg/mol. N2O
+
+	tile_overlay = "sleeping_agent"
+	overlay_limit = 1
+
+/decl/xgm_gas/oxygen_agent_b
+	id = "oxygen_agent_b"
+	name = "Oxygen Agent-B"	//what is this?
+	specific_heat = 300	// J/(mol*K)
+	molar_mass = 0.032	// kg/mol