Fix indexing

Author: SabineHaas

windpowerlib/power_curves.py

@@ -130,12 +130,12 @@ def smooth_power_curve(power_curve_wind_speeds, power_curve_values,
     # Initialize list for power curve values
     smoothed_power_curve_values = []
     # Append wind speeds to `power_curve_wind_speeds`
-    maximum_value = power_curve_wind_speeds.values[-1] + wind_speed_range
+    maximum_value = power_curve_wind_speeds.iloc[-1] + wind_speed_range
     while power_curve_wind_speeds.values[-1] < maximum_value:
         power_curve_wind_speeds = power_curve_wind_speeds.append(
             pd.Series(power_curve_wind_speeds.iloc[-1] +
-                      (power_curve_wind_speeds[5] -
-                       power_curve_wind_speeds[4]),
+                      (power_curve_wind_speeds.iloc[5] -
+                       power_curve_wind_speeds.iloc[4]),
                       index=[power_curve_wind_speeds.index[-1] + 1]))
         power_curve_values = power_curve_values.append(
             pd.Series(0.0, index=[power_curve_values.index[-1] + 1]))