Add modules to init

Author: birgits

windpowerlib/__init__.py

@@ -5,10 +5,14 @@
 from windpowerlib import density
 from windpowerlib import modelchain
 from windpowerlib import power_output
+from windpowerlib import power_curve
 from windpowerlib import temperature
 from windpowerlib import tools
 from windpowerlib import wind_speed
 from windpowerlib import wind_turbine
+from windpowerlib import wind_farm
+from windpowerlib import turbine_cluster_modelchain
+from windpowerlib import wind_turbine_cluster
 
 from windpowerlib.wind_turbine import WindTurbine
 from windpowerlib.modelchain import ModelChain