ENH: consolidate decomposition model, constructors for decomposition methods

- make regionName an optional constructor parameter, which eliminates
  a separate set of constructors and construction tables. Adjust
  internals to treat a missing/empty regionName as a no-op.

- pass in fallback dictionary content via new IOdictionary constructor
  with a pointer

ENH: further relax check for matching number of processor dirs

- if the "numberOfSubdomains" entry is missing (or even zero)
  ignore checks of processor dirs as meaningless.

src/parallel/decompose/decompositionMethods/structuredDecomp/structuredDecomp.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2012 OpenFOAM Foundation
-    Copyright (C) 2018 OpenCFD Ltd.
+    Copyright (C) 2018-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -52,7 +52,7 @@ class structuredDecomp
 :
     public decompositionMethod
 {
-    // Private data
+    // Private Data
 
         dictionary methodDict_;
 
@@ -78,8 +78,12 @@ public:
 
     // Constructors
 
-        //- Construct given the decomposition dictionary
-        structuredDecomp(const dictionary& decompDict);
+        //- Construct given decomposition dictionary. Region ignored
+        explicit structuredDecomp
+        (
+            const dictionary& decompDict,
+            const word& regionName = ""
+        );
 
 
     //- Destructor