functionObjects: rewritten to all be derived from 'functionObject'

- Avoids the need for the 'OutputFilterFunctionObject' wrapper
  - Time-control for execution and writing is now provided by the
    'timeControlFunctionObject' which instantiates the processing
    'functionObject' and controls its operation.
  - Alternative time-control functionObjects can now be written and
    selected at run-time without the need to compile wrapped version of
    EVERY existing functionObject which would have been required in the
    old structure.
  - The separation of 'execute' and 'write' functions is now formalized in the
    'functionObject' base-class and all derived classes implement the
    two functions.
  - Unnecessary implementations of functions with appropriate defaults
    in the 'functionObject' base-class have been removed reducing
    clutter and simplifying implementation of new functionObjects.
  - The 'coded' 'functionObject' has also been updated, simplified and tested.
  - Further simplification is now possible by creating some general
    intermediate classes derived from 'functionObject'.

tutorials/basic/potentialFoam/cylinder/system/controlDict

@@ -17,19 +17,19 @@ FoamFile
 
 application     potentialFoam;
 
-startFrom       startTime;
+startFrom        latestTime;
 
 startTime       0;
 
-stopAt          endTime;
+stopAt           nextWrite;
 
 endTime         1;
 
 deltaT          1;
 
-writeControl    timeStep;
+writeControl     timeStep;
 
-writeInterval   1;
+writeInterval    1;
 
 purgeWrite      0;
 
@@ -51,7 +51,11 @@ functions
     {
         // Load the library containing the 'coded' functionObject
         functionObjectLibs ("libutilityFunctionObjects.so");
+
         type coded;
+
+        writeControl timeStep;
+
         // Name of on-the-fly generated functionObject
         redirectType error;
         code