BUG: scaleFactor for fieldValue function-objects is 'sticky' (issue #331)

- if set in a dictionary and later removed, the intermediate value is
  retained rather than reverting to a normal default value.

- same applies for a few other function objects and their variables

src/functionObjects/field/fieldValues/fieldValue/fieldValue.C

@@ -97,7 +97,7 @@ bool Foam::functionObjects::fieldValue::read(const dictionary& dict)
 
     dict.lookup("fields") >> fields_;
     dict.lookup("writeFields") >> writeFields_;
-    dict.readIfPresent("scaleFactor", scaleFactor_);
+    scaleFactor_ = dict.lookupOrDefault<scalar>("scaleFactor", 1.0);
 
     return true;
 }

src/functionObjects/field/fieldValues/fieldValue/fieldValue.H

@@ -70,7 +70,7 @@ protected:
 
     // Protected data
 
-        //- Optional scale value
+        //- Optional scaling factor
         scalar scaleFactor_;
 
         //- Construction dictionary