STYLE: make polyMesh constructor explicit

- use polyMesh::defaultRegion instead of fvMesh::defaultRegion and
  others via static inheritance. (92 vs ~25 occurrences)

applications/utilities/preProcessing/changeDictionary/changeDictionary.C

@@ -499,7 +499,7 @@ int main(int argc, char *argv[])
 
 
         fileName regionPrefix;
-        if (regionName != fvMesh::defaultRegion)
+        if (regionName != polyMesh::defaultRegion)
         {
             regionPrefix = regionName;
         }