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

src/functionObjects/field/fieldsExpression/fieldsExpression.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2012-2016 OpenFOAM Foundation
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2020 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -31,13 +31,38 @@ Group
     grpFieldFunctionObjects
 
 Description
-    Intermediate class for handling field expressions (add, subtract etc).
+    Intermediate class for handling field expression function objects
+    (e.g. \c add, \c subtract etc.) whereinto more than one fields are input.
+
+Usage
+    Minimal example by using \c system/controlDict.functions:
+    \verbatim
+    <userDefinedSubDictName1>
+    {
+        // Mandatory and other optional entries
+        ...
+
+        // Optional (inherited) entries (runtime modifiable)
+        fields            (<field1> <field2> ... <fieldN>);
+        result            <fieldResult>;
+    }
+    \endverbatim
+
+    where the entries mean:
+    \table
+      Property  | Description                 | Type     | Req'd | Dflt
+      fields    | Names of operand fields     | wordList |  yes  | -
+      result    | Names of output fields | wordList | no | \<FO\>(\<f1\>,...)
+    \endtable
 
 See also
-    Foam::functionObjects::fvMeshFunctionObject
+    - Foam::functionObject
+    - Foam::functionObjects::fvMeshFunctionObject
+    - ExtendedCodeGuide::functionObjects::field::fieldsExpression
 
 SourceFiles
     fieldsExpression.C
+    fieldsExpressionTemplates.C
 
 \*---------------------------------------------------------------------------*/
 
@@ -64,7 +89,7 @@ class fieldsExpression
 {
 protected:
 
-    // Protected member data
+    // Protected Member Data
 
         //- Names of fields to process
         wordList fieldNames_;
@@ -98,12 +123,6 @@ protected:
         //- Calculate expression
         virtual bool calc() = 0;
 
-        //- No copy construct
-        fieldsExpression(const fieldsExpression&) = delete;
-
-        //- No copy assignment
-        void operator=(const fieldsExpression&) = delete;
-
 
 public:
 
@@ -123,6 +142,12 @@ public:
             const word& resultName = word::null
         );
 
+        //- No copy construct
+        fieldsExpression(const fieldsExpression&) = delete;
+
+        //- No copy assignment
+        void operator=(const fieldsExpression&) = delete;
+
 
     //- Destructor
     virtual ~fieldsExpression() = default;