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

tutorials/lagrangian/coalChemistryFoam/simplifiedSiwek/system/dataCloud

@@ -1,5 +1,6 @@
 // -*- C++ -*-
 // Minimal example of using the dataCloud function object.
+
 dataCloud
 {
     type    dataCloud;

tutorials/lagrangian/reactingParcelFoam/filter/system/controlDict

@@ -53,6 +53,7 @@ maxDeltaT       1;
 
 functions
 {
+    #include "dataCloud"
     #include "vtkCloud"
     #include "vtkWrite"
 }

tutorials/lagrangian/reactingParcelFoam/filter/system/dataCloud

@@ -0,0 +1,42 @@
+// -*- C++ -*-
+
+dataCloud
+{
+    type    dataCloud;
+    libs    ("liblagrangianFunctionObjects.so");
+    log     true;
+
+    // Nothing happens before this anyhow
+    timeStart 0.5;
+
+    writeControl    writeTime;
+
+    // cloud   reactingCloud1;
+    clouds  ( ".*" );
+
+    // Field to output
+    field   d;
+
+    // Optional selection mechanism
+    selection
+    {
+        // Reduced number of output parcels
+        stride
+        {
+            action  add;
+            source  stride;
+            stride  4;
+        }
+
+        Umin
+        {
+            action  subtract;
+            source  field;
+            field   U;
+            accept  (less 0.2);
+        }
+    }
+}
+
+
+// ************************************************************************* //

tutorials/lagrangian/reactingParcelFoam/filter/system/vtkCloud

@@ -28,6 +28,7 @@ cloudWrite
     //- Output directory name - Default postProcessing
     // directory       "VTK";
 
+    // Optional selection mechanism
     selection
     {
         all