ENH: IOobject::selectIO helper method

- centralizes IOobject handling and treatment of alternative locations.
  If an alternative file location is specified, it will be used instead.

- provide decompositionMethod::canonicalName instead of using
  "decomposeParDict" in various places.

src/OpenFOAM/include/setSystemMeshDictionaryIO.H

@@ -1,28 +1,12 @@
-fileName dictPath;
-if (args.readIfPresent("dict", dictPath))
-{
-    if (isDir(dictPath))
-    {
-        dictPath = dictPath / dictName;
-    }
-}
-
-IOobject dictIO
+IOobject dictIO = IOobject::selectIO
 (
-    dictName,
-    runTime.system(),
-    mesh,
-    IOobject::MUST_READ_IF_MODIFIED,
-    IOobject::NO_WRITE
-);
-
-if (dictPath.size())
-{
-    dictIO = IOobject
+    IOobject
     (
-        dictPath.expand(),
+        dictName,
+        runTime.system(),
         mesh,
         IOobject::MUST_READ_IF_MODIFIED,
         IOobject::NO_WRITE
-    );
-}
+    ),
+    args.lookupOrDefault<fileName>("dict", "")
+);