Add missing setter for installed power

Author: birgits

windpowerlib/wind_farm.py

@@ -111,6 +111,10 @@ def installed_power(self):
             FutureWarning)
         return self.nominal_power
 
+    @installed_power.setter
+    def installed_power(self, installed_power):
+        self._installed_power = installed_power
+
     @property
     def nominal_power(self):
         r"""

windpowerlib/wind_turbine_cluster.py

@@ -75,6 +75,10 @@ def installed_power(self):
             FutureWarning)
         return self.nominal_power
 
+    @installed_power.setter
+    def installed_power(self, installed_power):
+        self._installed_power = installed_power
+
     @property
     def nominal_power(self):
         r"""