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/postProcessing/dataConversion/foamToVTK/foamToVTK.C

@@ -633,7 +633,8 @@ int main(int argc, char *argv[])
     else
     {
         regionNames.resize(1);
-        regionNames.first() = args.get<word>("region", fvMesh::defaultRegion);
+        regionNames.first() =
+            args.getOrDefault<word>("region", fvMesh::defaultRegion);
     }
 
 
@@ -695,7 +696,7 @@ int main(int argc, char *argv[])
     // Directory management
 
     // Sub-directory for output
-    const word vtkDirName = args.get<word>("name", "VTK");
+    const word vtkDirName = args.getOrDefault<word>("name", "VTK");
 
     const fileName outputDir(args.globalPath()/vtkDirName);