Add GroundModel and improve WindTurbines.plot_yz

docs/notebooks/EngineeringWindFarmModels.ipynb

py_wake/deficit_models/gaussian.py

py_wake/deficit_models/gcl.py

py_wake/deficit_models/noj.py

py_wake/flow_map.py

py_wake/ground_models/__init__.py

py_wake/ground_models/ground_models.py

py_wake/tests/test_deficit_models/test_gcl.py

py_wake/tests/test_flow_map.py

py_wake/tests/test_ground_models/__init__.py

py_wake/tests/test_ground_models/test_mirror.py

py_wake/tests/test_windturbines.py

py_wake/wind_farm_models/engineering_models.py

py_wake/wind_farm_models/wind_farm_model.py

py_wake/wind_turbines.py