fix and test gradients of SimpleYawModel, DensityScale, PowerCtNDTabular,...

py_wake/deficit_models/fuga.py

py_wake/site/distance.py

py_wake/site/xrsite.py

py_wake/tests/test_utils/test_gradient_utils.py

py_wake/tests/test_utils/test_model_gradients.py

py_wake/tests/test_windturbines/test_power_ct_curves.py

py_wake/utils/functions.py

py_wake/utils/gradients.py

py_wake/utils/grid_interpolator.py

py_wake/wind_turbines/power_ct_functions.py