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.

applications/utilities/surface/surfaceMeshConvert/surfaceMeshConvert.C

@@ -168,7 +168,7 @@ int main(int argc, char *argv[])
                 IOobject::NO_WRITE,
                 false
             ),
-            args.opt<fileName>("dict", "")
+            args.get<fileName>("dict", "")
         );
 
         if (!ioCsys.typeHeaderOk<coordinateSystems>(false))