Repository: wind-python/windpowerlib

Issue:
Creating unphysical instances of WindTurbine class
With the current API nothing prevents a user from creating a nonphysical wind turbine instance, i.e., wind turbine that has *hub_height* shorter than 0.5*_rotor_diameter_. I would suggest raising ```ValueError``` with a useful message in case this happens, what do you think?

I wrote a small fix that passes all the existing test and adds new. If this is something you would like to review, and see fixed, I could easily put it in a PR?

Discussion:

Patch:
diff --git a/windpowerlib/wind_turbine.py b/windpowerlib/wind_turbine.py
index 91668e6..851ea30 100644
--- a/windpowerlib/wind_turbine.py
+++ b/windpowerlib/wind_turbine.py
@@ -163,6 +163,11 @@ class WindTurbine(object):
                 if self.rotor_diameter is None and turbine_data is not None:
                     self.rotor_diameter = float(turbine_data['rotor_diameter'])
 
+        if self.rotor_diameter:
+            if self.hub_height <= 0.5*self.rotor_diameter:
+                msg = "1/2rotor_diameter cannot be greater than hub_height"
+                raise ValueError(msg)
+
         if self.power_curve is None and self.power_coefficient_curve is None:
             msg = ("The WindTurbine has been initialised without a power curve"
                    " and without a power coefficient curve.\nYou will not be"
@@ -486,7 +491,7 @@ def get_turbine_types(turbine_library='local', print_out=True, filter_=True):
     turbine type itself) is missing you can contact us via github or
     windpowerlib@rl-institut.de. You can help us by providing data in the
     format as shown in
-    `the data base <https://openenergy-platform.org/dataedit/view/supply/wind_turbine_library>`_.
+    `the data base <https://openenergy-platform.org/dataedit/view/model_draft/openfred_windpower_powercurve>`_.
 
     Examples
     --------


Test patch:
diff --git a/tests/test_wind_turbine.py b/tests/test_wind_turbine.py
index 5ba627a..c876620 100644
--- a/tests/test_wind_turbine.py
+++ b/tests/test_wind_turbine.py
@@ -97,3 +97,14 @@ class TestWindTurbine:
                            match="The 'number' and the 'total_capacity' "
                                  "parameter are mutually exclusive."):
             e_t_1.to_group(5, 3000)
+
+    def test_create_unphysical_turbine(self):
+        err_msg = "1/2rotor_diameter cannot be greater than hub_height"
+        char = {
+           'hub_height': 80,
+           'rotor_diameter': 160,
+           'turbine_type': 'DUMMY 3',
+           'path': self.source
+        }
+        with pytest.raises(ValueError, match=err_msg):
+            WindTurbine(**char)