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/solvers/compressible/rhoSimpleFoam/overRhoSimpleFoam/createUpdatedDynamicFvMesh.H

@@ -1,22 +1,52 @@
-    Info<< "Create dynamic mesh for time = "
-        << runTime.timeName() << nl << endl;
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2017 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM, distributed under GPL-3.0-or-later.
 
-    autoPtr<dynamicFvMesh> meshPtr
+Description
+    Create a dynamicFvMesh, with init()
+
+Required Variables
+    - runTime [Time]
+
+Provided Variables
+    - mesh [dynamicFvMesh], meshPtr
+
+\*---------------------------------------------------------------------------*/
+
+Foam::autoPtr<Foam::dynamicFvMesh> meshPtr;
+
+{
+    Foam::Info << "Create dynamic mesh for time = "
+        << runTime.timeName() << Foam::nl;
+
+    meshPtr = dynamicFvMesh::New
     (
-        dynamicFvMesh::New
+        IOobject
         (
-            IOobject
-            (
-                polyMesh::defaultRegion,
-                runTime.timeName(),
-                runTime,
-                IOobject::MUST_READ
-            )
+            polyMesh::defaultRegion,
+            runTime.timeName(),
+            runTime,
+            IOobject::MUST_READ
         )
     );
+}
 
-    dynamicFvMesh& mesh = meshPtr();
 
-    // Calculate initial mesh-to-mesh mapping. Note that this should be
-    // done under the hood, e.g. as a MeshObject
-    mesh.update();
+dynamicFvMesh& mesh = meshPtr();
+
+// Calculate initial mesh-to-mesh mapping. Note that this should be
+// done under the hood, e.g. as a MeshObject
+mesh.update();
+
+Foam::Info << Foam::endl;
+
+
+// ************************************************************************* //