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/preProcessing/changeDictionary/changeDictionary.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2021 OpenCFD Ltd.
+    Copyright (C) 2016-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -498,13 +498,6 @@ int main(int argc, char *argv[])
         }
 
 
-        fileName regionPrefix;
-        if (regionName != polyMesh::defaultRegion)
-        {
-            regionPrefix = regionName;
-        }
-
-
         // Make sure we do not use the master-only reading since we read
         // fields (different per processor) as dictionaries.
         IOobject::fileModificationChecking = IOobject::timeStamp;
@@ -547,7 +540,7 @@ int main(int argc, char *argv[])
                 "boundary",
                 runTime.findInstance
                 (
-                    regionPrefix/polyMesh::meshSubDir,
+                    polyMesh::regionName(regionName)/polyMesh::meshSubDir,
                     "boundary",
                     IOobject::READ_IF_PRESENT
                 ),