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/mapFields/setTimeIndex.H

@@ -11,7 +11,7 @@
             sourceTimeIndex = Time::findClosestTimeIndex
             (
                 sourceTimes,
-                args.opt<scalar>("sourceTime")
+                args.get<scalar>("sourceTime")
             );
         }
     }