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/mesh/conversion/plot3dToFoam/plot3dToFoam.C

@@ -96,7 +96,7 @@ int main(int argc, char *argv[])
          FatalError.exit();
     }
 
-    const scalar scaleFactor = args.opt<scalar>("scale", 1);
+    const scalar scaleFactor = args.get<scalar>("scale", 1);
 
     const bool readBlank = !args.found("noBlank");
     const bool singleBlock = args.found("singleBlock");