Adding Python examples for VTK file formats.

src/Cxx.md

src/Python.md

src/Python/GeometricObjects/IsoparametricCellsDemo.md

src/Python/GeometricObjects/IsoparametricCellsDemo.py

src/Python/GeometricObjects/LinearCellDemo.md

src/Python/GeometricObjects/LinearCellDemo.py

src/Python/IO/ReadLegacyUnstructuredGrid.md

src/Python/IO/ReadLegacyUnstructuredGrid.py

src/Python/IO/WriteLegacyLinearCells.md

src/Python/IO/WriteLegacyLinearCells.py

src/Python/IO/WriteXMLLinearCells.md

src/Python/IO/WriteXMLLinearCells.py