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/surface/surfaceBooleanFeatures/surfaceBooleanFeatures.C

@@ -1596,7 +1596,7 @@ int main(int argc, char *argv[])
 
 
     // Scale factor for both surfaces:
-    const scalar scaleFactor = args.opt<scalar>("scale", -1);
+    const scalar scaleFactor = args.get<scalar>("scale", -1);
 
     const word surf1Name(args[2]);
     Info<< "Reading surface " << surf1Name << endl;