Making DataSetCellIterators have proper point data type.

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.