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/surfaceInertia/surfaceInertia.C

@@ -89,7 +89,7 @@ int main(int argc, char *argv[])
     argList args(argc, argv);
 
     const fileName surfFileName = args[1];
-    const scalar density = args.opt<scalar>("density", 1);
+    const scalar density = args.get<scalar>("density", 1);
 
     vector refPt = Zero;
     bool calcAroundRefPt = args.readIfPresent("referencePoint", refPt);