BUG: vtkCloud suppresses output for empty clouds (closes #866)

- the original intention was to avoid vtp output when the clouds are
  empty anyhow. However, it is useful for post-processing to have
  clouds with zero parcels (eg, before the start of injection).

  Pruning of empty clouds is now an option in the vtkCloud dictionary
  controls, with the default being false (no pruning). This represents
  a non-breaking change in behaviour since it generates more output
  than previously.

  Example,
  {
      type    vtkCloud;
      //- Suppress writing of empty clouds (default: false)
      prune   true;
  }

tutorials/lagrangian/reactingParcelFoam/filter/Allrun-parallel

@@ -0,0 +1,19 @@
+#!/bin/sh
+cd ${0%/*} || exit 1                        # Run from this directory
+. $WM_PROJECT_DIR/bin/tools/RunFunctions    # Tutorial run functions
+
+restore0Dir
+
+runApplication blockMesh
+
+runApplication topoSet
+
+runApplication createBaffles -overwrite
+
+runApplication decomposePar
+
+runParallel $(getApplication)
+
+runApplication reconstructPar
+
+#------------------------------------------------------------------------------

tutorials/lagrangian/reactingParcelFoam/filter/system/controlDict

@@ -51,5 +51,9 @@ maxCo           1.0;
 
 maxDeltaT       1;
 
+functions
+{
+    #include "vtkCloud"
+}
 
 // ************************************************************************* //

tutorials/lagrangian/reactingParcelFoam/filter/system/decomposeParDict

@@ -26,10 +26,7 @@ preserveFaceZones
 
 coeffs
 {
-    n           (2 2 1);
-    //delta       0.001; // default=0.001
-    //order       xyz;   // default=xzy
-    dataFile    "";
+    n       (2 2 1);
 }
 
 // ************************************************************************* //

tutorials/lagrangian/reactingParcelFoam/filter/system/vtkCloud

@@ -0,0 +1,31 @@
+// -*- C++ -*-
+
+cloudWrite
+{
+    type    vtkCloud;
+    libs    ("liblagrangianFunctionObjects.so");
+    log     true;
+
+    writeControl    writeTime;
+
+    // cloud   reactingCloud1;
+    clouds  ( ".*" );
+
+    // Fields to output (words or regex)
+    fields  ( U T d "Y.*" );
+
+    //- Output format (ascii | binary) - default = binary
+    // format  binary;
+
+    // format   ascii;
+    // writePrecision 12;
+
+    //- Suppress writing of empty clouds - default = false
+    // prune   true;
+
+    //- Output directory name - default = "VTK"
+    // directory       "VTK";
+}
+
+
+// ************************************************************************* //