Clean up AEPCalculator and WakeModel interface

_notebooks/elements/BastankhahGaussian.ipynb

_notebooks/elements/IEA37SimpleBastankhahGaussian.ipynb

_notebooks/elements/cmp_wakemodels.ipynb

_notebooks/elements/fuga.ipynb

...ooks/elements/hornsrev_layout_optimization_exercise.ipynb

_notebooks/elements/noj.ipynb

_notebooks/elements/noj_validation_exercise.ipynb

_notebooks/elements/noj_validation_exercise_solution.ipynb

_notebooks/notebook.py

docs/notebooks/BastankhahGaussian.ipynb

docs/notebooks/IEA37SimpleBastankhahGaussian.ipynb

docs/notebooks/cmp_wakemodels.ipynb

docs/notebooks/fuga.ipynb

docs/notebooks/noj.ipynb

docs/source/validation_report/tests.py

py_wake/aep_calculator.py

py_wake/examples/hornsrev1_example.py

py_wake/examples/iea37_example.py

py_wake/site/_site.py

py_wake/site/wasp_grid_site.py