Adding KochanekSplineDemo, improving the spline widgets

src/Cxx/PolyData/KochanekSplineDemo.md

src/PythonicAPI.md

src/PythonicAPI/PolyData/KochanekSplineDemo.md

src/PythonicAPI/PolyData/KochanekSplineDemo.py

src/PythonicAPI/Snippets/Slider2D.md

src/PythonicAPI/Snippets/Slider3D.md

src/PythonicAPI/Widgets/Slider2D.py

src/PythonicAPI/Widgets/Slider3D.py

.../Baseline/PythonicAPI/PolyData/TestKochanekSplineDemo.png