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/surface/surfaceConvert/surfaceConvert.C

@@ -149,7 +149,7 @@ int main(int argc, char *argv[])
     argList args(argc, argv);
 
     {
-        const unsigned prec = args.lookupOrDefault<unsigned>("precision", 0u);
+        const unsigned prec = args.getOrDefault<unsigned>("precision", 0u);
         if (prec)
         {
             Info<< "Output write precision set to " << prec << endl;