REFPROP backend can calculate molar mass

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>

src/Backends/REFPROP/REFPROPMixtureBackend.h

@@ -49,6 +49,13 @@ public:
 				double value2
 				);
 
+    long double calc_molar_mass(void){
+        if (!_molar_mass){
+            calc_molar_mass();
+        }
+        return _molar_mass.pt();
+    };
+
 	/// Returns true if REFPROP is supported on this platform
 	bool REFPROP_supported(void);