This commit brings HTG API closer to what VTK is expecting for Dimensions and Extent.
By that we mean that GridSize/GridExtent has been replaced by Dimensions/Extent which now represent the number of points along each axis.
We also assume HTG extent always start with 0.
Many tests have also been added but are currently missing baseline image comparison.

This code has been contributed by the CEA (Jacques-Bernard Lekien and Jerome Dubois)