For accuracy the vtkDataSetCellIterator should have
the same floating point accuracy as the input data set. For
ImageData and HyperOctree it will be double since that is
their native data type for storing point locations. For
RectilinearGrid and HyperTreeGrid we do the best we can
but there are 3 arrays which store the point locations
for these grids so it's not always clear which one to use.
This was already done for the other data set cell iterators.