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/preProcessing/createZeroDirectory/createZeroDirectory.C

@@ -252,7 +252,7 @@ int main(int argc, char *argv[])
 
     fileName baseDir
     (
-        args.opt<fileName>
+        args.get<fileName>
         (
             "templateDir",
             "${WM_PROJECT_DIR}/etc/caseDicts/createZeroDirectoryTemplates"