fix gradients of TerrainFollowingDistance + gridded site with multiple ws

py_wake/site/_site.py

py_wake/site/distance.py

py_wake/site/xrsite.py

py_wake/tests/test_sites/test_distances.py

py_wake/tests/test_sites/test_xrsite.py

py_wake/tests/test_utils/test_gradient_utils.py

py_wake/tests/test_utils/test_model_gradients.py

py_wake/utils/gradients.py

py_wake/utils/grid_interpolator.py

py_wake/utils/xarray_utils.py