ENH: support predicate checks for argList (similar to dictionary methods)

- Favour use of argList methods that are more similar to dictionary
  method names with the aim of reducing the cognitive load.

  * Silently deprecate two-parameter get() method in favour of the
    more familiar getOrDefault.
  * Silently deprecate opt() method in favour of get()

  These may be verbosely deprecated in future versions.

applications/utilities/preProcessing/createZeroDirectory/createZeroDirectory.C

@@ -250,7 +250,7 @@ int main(int argc, char *argv[])
 
     fileName baseDir
     (
-        args.get<fileName>
+        args.getOrDefault<fileName>
         (
             "templateDir",
             "${WM_PROJECT_DIR}/etc/caseDicts/createZeroDirectoryTemplates"