add GCL wake model and RotorAvgModel

py_wake/__init__.py

py_wake/deficit_models/fuga.py

py_wake/deficit_models/gaussian.py

py_wake/deficit_models/gcl.py

py_wake/rotor_avg_models/__init__.py

py_wake/rotor_avg_models/rotor_avg_model.py

py_wake/tests/test_deficit_models/test_gcl.py

py_wake/tests/test_deficit_models/test_rotor_avg_models.py

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

.../test_wind_farm_models/test_enginering_wind_farm_model.py

py_wake/wind_farm_models/engineering_models.py