Adding VelocityProfile.

src/Cxx.md

src/Cxx/VisualizationAlgorithms/CMakeLists.txt

src/Cxx/VisualizationAlgorithms/VelocityProfile.cxx

src/Cxx/VisualizationAlgorithms/VelocityProfile.md

src/Python.md

src/Python/VisualizationAlgorithms/VelocityProfile.md

src/Python/VisualizationAlgorithms/VelocityProfile.py