Revert changes

Author: birgits

windpowerlib/wind_farm.py

@@ -361,7 +361,7 @@ def assign_power_curve(self, wake_losses_model='wind_farm_efficiency',
                 if power_curve.iloc[0]['wind_speed'] != 0.0:
                     power_curve = pd.concat(
                         [pd.DataFrame(data={
-                            'wind_speed': [0.0], 'value': [0.0]}),
+                            'value': [0.0], 'wind_speed': [0.0]}),
                             power_curve], join='inner')
                 if power_curve.iloc[-1]['value'] != 0.0:
                     power_curve = pd.concat(