vtkImageData and its subclasses now builds structured Points, CellArray, CellTypesArray.

These structures allow faster queries instead of the pre-existing switch-cases.
These structures should not be ShallowCopy/DeepCopy, since they will be automatically
be computed when SetExtent is called.

Additionally, add GetPoints, GetCells and GetCellTypesArray functions.

Lastly, visibility related checks, and blanking functions have been moved from vtkUniformGrid
to vtkImageData.