DOC: elaborate the usage of function objects

ENH: update libs of etc/caseDicts/postProcess items
  ENH: ensure destructor=default
  ENH: ensure constness
  ENH: ensure no 'copy construct' and 'no copy assignment' exist
  TUT: add examples of function objects with full set
       of settings into a TUT if unavailable
  TUT: update pisoFoam/RAS/cavity tutorial in terms of usage

tutorials/multiphase/twoPhaseEulerFoam/laminar/bubbleColumn/system/controlDict

@@ -54,6 +54,15 @@ maxDeltaT       1;
 
 functions
 {
+    add1
+    {
+        type            add;
+        libs            (fieldFunctionObjects);
+        fields          (U.air U.water);
+        result          U.total;
+        writeControl    writeTime;
+    }
+
     fieldAverage1
     {
         type            fieldAverage;