STYLE: argList::opt method instead of the longer argList::lookupOrDefault

- also replaced a few instances of readIfPresent with opt<> for
  constant values.

src/OpenFOAM/include/setSystemMeshDictionaryIO.H

@@ -8,5 +8,5 @@ IOobject dictIO = IOobject::selectIO
         IOobject::MUST_READ_IF_MODIFIED,
         IOobject::NO_WRITE
     ),
-    args.lookupOrDefault<fileName>("dict", "")
+    args.opt<fileName>("dict", "")
 );