ENH: output filtering of vtkCloud by user selection (#1056)

- can filter by stride or field information.
  For example,

      selection
      {
          stride
          {
              // every 10th parcelId
              action  add;
              source  stride;
              stride  10;
          }
          Umin
          {
              // Remove slow parcels
              action  subtract;
              source  field;
              field   U;
              accept  (less 1e-3);
          }
          diam
          {
              // Only particular diameter ranges
              action  subset;
              source  field;
              field   d;
              accept  (greater 1e-3) and (less 1e-3);
          }
      }