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

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

applications/utilities/parallelProcessing/decomposePar/decomposePar.C

@@ -327,8 +327,7 @@ int main(int argc, char *argv[])
 
 
     // Allow override of decomposeParDict location
-    fileName decompDictFile;
-    args.readIfPresent("decomposeParDict", decompDictFile);
+    const fileName decompDictFile = args.opt<fileName>("decomposeParDict", "");
 
     // Get all region names
     wordList regionNames;
@@ -342,8 +341,7 @@ int main(int argc, char *argv[])
     else
     {
         regionNames.resize(1);
-        regionNames.first() =
-            args.lookupOrDefault<word>("region", fvMesh::defaultRegion);
+        regionNames.first() = args.opt<word>("region", fvMesh::defaultRegion);
     }
 
     forAll(regionNames, regioni)