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/checkMesh/checkGeometry.C

@@ -1015,10 +1015,8 @@ Foam::label Foam::checkGeometry
 
         const fileName outputDir
         (
-            mesh.time().globalPath()
-           /functionObject::outputPrefix
-           /(mesh.name() == polyMesh::defaultRegion ? word::null : mesh.name())
-           /"checkMesh"
+            mesh.time().globalPath()/functionObject::outputPrefix
+          / mesh.regionName()/"checkMesh"
         );
 
         forAll(pbm, patchi)

applications/utilities/mesh/manipulation/checkMesh/checkTools.C

@@ -52,15 +52,8 @@ License
 
 void Foam::printMeshStats(const polyMesh& mesh, const bool allTopology)
 {
-    if (mesh.name() == Foam::polyMesh::defaultRegion)
-    {
-        Info<< "Mesh stats" << nl;
-    }
-    else
-    {
-        Info<< "Mesh " << mesh.name() << " stats" << nl;
-    }
-    Info<< "    points:           "
+    Info<< "Mesh stats " << mesh.regionName() << nl
+        << "    points:           "
         << returnReduce(mesh.points().size(), sumOp<label>()) << nl;
 
 

applications/utilities/mesh/manipulation/rotateMesh/rotateMesh.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2019-2021 OpenCFD Ltd.
+    Copyright (C) 2019-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -130,11 +130,10 @@ 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)
         {

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)
         {