Ported GradientFilter from Cxx to Python

[FlorianFritz]

Plain port from Cxx to Python

[ajpmaclean]

This return statement is not needed.

[ajpmaclean]

Looks good. Here are some extra things you will need to do:

1. In the file src/Python.md section (Working with 3D Data) you will need to add the following line in alphabetical order to:

[GradientFilter](/Python/PolyData/GradientFilter) | vtkGradientFilter | Compute the gradient of a scalar field on a data set.

2. You will need to copy src/Testing/Baseline/Cxx/PolyData/TestGradientFilter.png to src/Testing/Baseline/Python/PolyData/

3. Consider using vtkNamedColors to set the colors. See GeometricObjectsDemo for how to do this. In this case, use 84, 89, 109 as the rgb values.

Steps 1, 2 ensure that the code appears on the web page in the correct section with a nice picture.