Make "GroundModel" a DeficitModel attribute

docs/notebooks/Site.ipynb

docs/notebooks/YawMisalignment.ipynb

docs/notebooks/exercises/CombineModels.ipynb

docs/notebooks/exercises/Validation.ipynb

py_wake/deficit_models/__init__.py

py_wake/deficit_models/deficit_model.py

py_wake/deficit_models/fuga.py

py_wake/deficit_models/gaussian.py

py_wake/deficit_models/gcl.py

py_wake/deficit_models/hybridinduction.py

py_wake/deficit_models/no_wake.py

py_wake/deficit_models/noj.py

py_wake/deficit_models/rankinehalfbody.py

py_wake/deficit_models/rathmann.py

py_wake/deficit_models/selfsimilarity.py

py_wake/deficit_models/vortexcylinder.py

py_wake/deficit_models/vortexdipole.py

py_wake/rotor_avg_models/rotor_avg_model.py

py_wake/tests/test_deficit_models/test_blockage_models.py

py_wake/tests/test_deficit_models/test_deficit_models.py