ENH: add 'filtered' polyMesh regionName() method

- in various situations with mesh regions it is also useful to
  filter out or remove the defaultRegion name (ie, "region0").

  Can now do that conveniently from the polyMesh itself or as a static
  function. Simply use this

      const word& regionDir = polyMesh::regionName(regionName);

  OR  mesh.regionName()

  instead of

      const word& regionDir =
      (
           regionName != polyMesh::defaultRegion
         ? regionName
         : word::null
      );

  Additionally, since the string '/' join operator filters out empty
  strings, the following will work correctly:

      (polyMesh::regionName(regionName)/polyMesh::meshSubDir)

      (mesh.regionName()/polyMesh::meshSubDir)

applications/utilities/mesh/manipulation/transformPoints/transformPoints.C

@@ -369,11 +369,10 @@ int main(int argc, char *argv[])
     forAll(regionNames, regioni)
     {
         const word& regionName = regionNames[regioni];
-        const word& regionDir =
+        const fileName meshDir
         (
-            regionName == polyMesh::defaultRegion ? word::null : regionName
+            polyMesh::regionName(regionName)/polyMesh::meshSubDir
         );
-        const fileName meshDir = regionDir/polyMesh::meshSubDir;
 
         if (regionNames.size() > 1)
         {