Documentation + make alpha and ti function of wd

.gitignore

docs/source/conf.py

docs/source/index.rst

docs/source/installation.rst

docs/source/tutorials.rst

py_wake/aep_calculator.py

py_wake/examples/data/hornsrev1.py

py_wake/examples/data/iea37/__init__.py

py_wake/site/__init__.py

py_wake/site/_site.py

py_wake/tests/test_site.py

py_wake/wake_model.py

py_wake/wake_models/fuga.py

py_wake/wake_models/gaussian.py

py_wake/wake_models/noj.py

py_wake/wind_turbines.py

tutorials/windturbines.ipynb