functionObjects: Added fields() function to provide list of required fields to postProcess

With this change each functionObject provides the list of fields required so
that the postProcess utility can pre-load them before executing the list of
functionObjects.  This provides a more convenient interface than using the
-field or -fields command-line options to postProcess which are now redundant.

src/functionObjects/utilities/codedFunctionObject/codedFunctionObject.C

@@ -59,6 +59,7 @@ Foam::wordList Foam::codedFunctionObject::codeKeys() const
         "codeExecute",
         "codeInclude",
         "codeRead",
+        "codeFields",
         "codeWrite",
         "localCode"
     };
@@ -152,6 +153,13 @@ Foam::functionObject& Foam::codedFunctionObject::redirectFunctionObject() const
 }
 
 
+Foam::wordList Foam::codedFunctionObject::fields() const
+{
+    updateLibrary();
+    return redirectFunctionObject().fields();
+}
+
+
 bool Foam::codedFunctionObject::execute()
 {
     updateLibrary();