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

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

applications/utilities/mesh/manipulation/mergeMeshes/mergeMeshes.C

@@ -103,18 +103,10 @@ int main(int argc, char *argv[])
     fileName addCase = args[2];
 
     const word masterRegion =
-        args.lookupOrDefault<word>
-        (
-            "masterRegion",
-            polyMesh::defaultRegion
-        );
+        args.opt<word>("masterRegion", polyMesh::defaultRegion);
 
     const word addRegion =
-        args.lookupOrDefault<word>
-        (
-            "addRegion",
-            polyMesh::defaultRegion
-        );
+        args.opt<word>("addRegion", polyMesh::defaultRegion);
 
     // Since we don't use argList processor directory detection, add it to
     // the casename ourselves so it triggers the logic inside TimePath.