Add an abstract vtkCartesianGrid

A new abstract class, vtkCartesianGrid add a layer of abstraction on both vtkImageData and vtkRectilinearGrid.

It is now possible to use a common API for both the classes.

No behavior were changed.

vtkRectilinearGrid::ComputeStructuredCoordinates(double*, ...) has been deprecated in favor or vtkRectilinearGrid::ComputeStructuredCoordinates(const double*, ...)

vtkImageData::SetExtent is not virtual anymore.

BuildImplicitStructures, BuildCells and BuildCellTypes have been privatized, they are called as needed bt the vtkCartesianGrid. Extent, Dimensions and DataDescription members have been privatized, use the accessors.