ENH: support predicate checks for argList (similar to dictionary methods)

- Favour use of argList methods that are more similar to dictionary
  method names with the aim of reducing the cognitive load.

  * Silently deprecate two-parameter get() method in favour of the
    more familiar getOrDefault.
  * Silently deprecate opt() method in favour of get()

  These may be verbosely deprecated in future versions.

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.get<scalar>("density", 1);
+    const scalar density = args.getOrDefault<scalar>("density", 1);
 
     vector refPt = Zero;
     bool calcAroundRefPt = args.readIfPresent("referencePoint", refPt);