ENH: provide operator[] for accessing argList options.

- operator[] with label -> get args at index
- operator[] with word  -> get named option

applications/utilities/preProcessing/mapFields/setTimeIndex.H

@@ -2,7 +2,7 @@
     label sourceTimeIndex = runTimeSource.timeIndex();
     if (args.optionFound("sourceTime"))
     {
-        if (args.option("sourceTime") == "latestTime")
+        if (args["sourceTime"] == "latestTime")
         {
             sourceTimeIndex = sourceTimes.size() - 1;
         }