functionObject: Log by default to stdout when in postProcess mode, not otherwise

This change formalises the usage of the "log" keyword in function
objects. By default, logging to stdout is activated when running
"postProcess" or "<solver> -postProcess", and deactivated when a
function is being executed as part of a run.

This behaviour can now be overridden in the function object dictionary
when operating in either mode.

src/OpenFOAM/db/functionObjects/functionObject/functionObject.C

@@ -126,9 +126,10 @@ const Foam::word& Foam::functionObject::name() const
 
 bool Foam::functionObject::read(const dictionary& dict)
 {
+    log = dict.lookupOrDefault<Switch>("log", postProcess);
+
     if (!postProcess)
     {
-        log = dict.lookupOrDefault<Switch>("log", true);
         executeAtStart_ = dict.lookupOrDefault<Switch>("executeAtStart", true);
     }