* 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 coordinates
* and cell to point indexing.
* Unlike unstructured grid, vtkExplicitStructuredGrid does not keep a cell type
* list as all cells are known to be hexahedra.
* vtkExplicitStructuredGrid can take advantage of its layout to perform operations
* based on the i, j, k parameters, similar to structured grid. This makes some
* operations faster on this class, without loosing the flexibility of the
* cell -> points mapping.
* The most common use of this class would be in situations where you have all
* hexahedra but the points used by the cells are not exactly defined by the
* i, j, k parameters. One example of this is a structured grid with a half voxel
* shift occurring in the middle of it such as with a geologic fault.
*
* The order and number of points is arbitrary.
* The order and number of cells must match that specified by the dimensions
* of the grid minus 1, because in vtk structured datasets the dimensions
* correspond to the points.
* The cells order increases in i fastest (from 0 <= i <= dims[0] - 2),
* then j (0 <= j <= dims[1] - 2), then k ( 0 <= k <= dims[2] - 2) where dims[]
* are the dimensions of the grid in the i-j-k topological directions.
* The number of cells is (dims[0] - 1) * (dims[1] - 1) * (dims[2] - 1).
*/
```