ENH: support 'get()' for retrieving argList options

- previously only had 'opt<..>()' for options, but 'get<..>()'
  provides more similarity with dictionary methods.
  The 'opt<..>()' method is retained.

src/OpenFOAM/include/setSystemMeshDictionaryIO.H

@@ -8,5 +8,5 @@ IOobject dictIO = IOobject::selectIO
         IOobject::MUST_READ_IF_MODIFIED,
         IOobject::NO_WRITE
     ),
-    args.opt<fileName>("dict", "")
+    args.get<fileName>("dict", "")
 );