ENH: add 'cellCentre' sampledSet (issue #869)

- samples on cell centres, can optionally limit based on a bounding box.
  Can be used, for example, to extract volume fields into CSV format.

tutorials/compressible/rhoSimpleFoam/squareBend/system/controlDict

@@ -53,6 +53,7 @@ functions
 {
     #include "sampling"
     // #include "samplingDebug"
+    // #include "sampleCellCentres"
 }
 
 

tutorials/compressible/rhoSimpleFoam/squareBend/system/sampleCellCentres

@@ -0,0 +1,34 @@
+// -*- C++ -*-
+
+sampleSets
+{
+    type    sets;
+    libs    ("libsampling.so");
+    log     on;
+    enabled true;
+
+    writeControl    timeStep;
+    writeInterval   10;
+
+    setFormat       vtk;
+    setFormat       csv;
+    interpolationScheme cell;
+
+    // fields ( U p );
+
+    fields ( U );
+
+    sets
+    (
+        centres
+        {
+            type    cellCentre;
+            // axis    xyz;  // default: xyz
+
+            // bounds  (-0.2 -1 -1) (-0.195 0 1);
+            bounds  (-0.025 -1 -1) (-0.0225 0 1);  // single cell layer
+        }
+    );
+}
+
+// ************************************************************************* //