Add Explicit Structured Grid

This MR includes a Python example for explicit structured grid. The example demonstrates how to

• create an explicit structured grid,
• convert an explicit structured grid to unstructured grid, and
• convert an unstructured grid to explicit structured grid.

The following VTK classes are used:

• vtkExplicitStructuredGrid,
• vtkExplicitStructuredGridToUnstructuredGrid and
• vtkUnstructuredGridToExplicitStructuredGrid.