ENH: improvements for dataCloud function object (issue #1044)

- now supports a parcel selection mechanism like vtkCloud,
  giving the ability to select a subset of parcels.
  For example, a given stride, or removal of parcels with a small
  diameter.

  Eg,
      dataCloud output Time: 3.2
      Applying parcel filtering to 994 parcels
      - add stride 4
      - subtract field U : (less 0.2)
      After filtering using 214/994 parcels

- add output precision control for dataCloud