bugfix and update of STF2005TurbulenceModel and STF2017TurbulenceModel

docs/notebooks/EngineeringWindFarmModels.ipynb

docs/notebooks/exercises/CombineModels.ipynb

py_wake/deficit_models/deficit_model.py

py_wake/superposition_models.py

py_wake/tests/test_deficit_models/test_deficit_models.py

py_wake/tests/test_rotor_avg_models.py

...ke/tests/test_turbulence_models/test_turbulence_models.py

py_wake/tests/test_utils/test_model_utils.py

py_wake/tests/test_windturbines/test_windturbines.py

py_wake/turbulence_models/__init__.py

py_wake/turbulence_models/crespo.py

py_wake/turbulence_models/gcl_turb.py

py_wake/turbulence_models/stf.py

py_wake/turbulence_models/turbulence_model.py

py_wake/utils/model_utils.py

py_wake/wind_farm_models/engineering_models.py

py_wake/wind_farm_models/wind_farm_model.py