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/tools/foamCalc/foamCalc.C

@@ -125,7 +125,7 @@ int main(int argc, char *argv[])
     }
 
     {
-        const int prec = args.lookupOrDefault<int>("precision", 0u);
+        const int prec = args.getOrDefault<int>("precision", 0u);
         if (prec > 0)
         {
             IOstream::defaultPrecision(prec);