Dont calculate density in ModelChain.run_model if not needed

Author: Birgit Schachler

windpowerlib/modelchain.py

@@ -345,6 +345,8 @@ def run_model(self, weather, data_height):
 
         """
         v_wind = self.v_wind_hub(weather, data_height)
-        rho_hub = self.rho_hub(weather, data_height)
+        rho_hub = None if (self.power_output_model=='p_values' and
+                           self.density_corr==False) \
+                       else self.rho_hub(weather, data_height)
         self.power_output = self.turbine_power_output(v_wind, rho_hub)
         return self