Clean up relative humidity function; closes #479

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

src/HumidAirProp.cpp

@@ -1233,10 +1233,9 @@ double RelativeHumidity(double T, double p, double psi_w)
 
     // Saturation pressure [Pa]
     p_s = f*p_ws;
-    // Find humidity ratio
-    W = HumidityRatio(psi_w);
-    // Find relative humidity using W/e=phi*p_s/(p-phi*p_s)
-    return W/epsilon*p/(p_s*(1+W/epsilon));
+    
+    // Calculate the relative humidity
+    return return psi_w*p/p_s;
 }
 
 void convert_to_SI(const std::string &Name, double &val)