ENH: add tmp/refPtr support for setting cref/ref from pointer

- makes it easier to use for local or alternative storage.
  Eg,

  ```
      tmp<volScalarField> tfld;

      tfld.cref(obj.cfindObject<volScalarField>("name"));

      if (!tfld)
      {
          tfld = volScalarField::New("name", ...);
      }
  ```

applications/test/refPtr/Test-refPtr.C

@@ -85,6 +85,12 @@ int main()
         ptr.reset(new scalarField(5, scalar(15)));
         tfld3.reset(std::move(ptr));
         printInfo(tfld3, true);
+
+
+        ptr.reset(new scalarField(2, scalar(1)));
+        Info<< nl << "const-ref from pointer: " << name(ptr.get()) << nl;
+        tfld3.cref(ptr.get());
+        printInfo(tfld3, true);
     }
 
     Info<< "\nEnd" << endl;