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.

applications/utilities/parallelProcessing/decomposePar/decomposePar.C

@@ -457,7 +457,7 @@ int main(int argc, char *argv[])
                         runTime,
                         IOobject::MUST_READ,
                         IOobject::NO_WRITE,
-                        false
+                        false       // do not register
                     ),
                     decompDictFile
                 )