Mathieu Westphal (Kitware) authored Jan 22, 2018 and Max Zeyen committed Feb 14, 2019

* The new class vtkExplicitStructuredGrid
* Two new filter vtkImageDataToExplicitStructuredGrid and vtkUnstructuredGridExplicitStructuredGrid for converting to the new type
* A new filter vtkExplicitStructuredGridToUnstructuredGrid for converting from the new type
* A new filter vtkExplicitStructuredGridCrop , as a first filter specific to the new type
* A few tests for these filters

```
 /* vtkExplicitStructuredGrid is a data object that is a concrete implementation
 * of vtkDataSet. vtkExplicitStructuredGrid represents a geometric structure
 * that is a topologically regular array of hexahedron. The topology is that of
 * a cube that has been subdivided into a regular array of smaller cubes.
 * Each cell can be addressed with i-j-k indices, however neighbor hexahedrons
 * does not necessarily share a face and hexahedron can be blanked (turned-off).
 *
 * Like unstructured grid, vtkExplicitStructuredGrid has explicit point coordinate...