- handle zero or negative values as being identical to 1.
  As per timeStep control and what the comments suggested.

- drop old outputTime enumeration, since this is covered by the
  writeTime enumeration and a corresponding Enum name.

- support construction of a "pass-through" control object that always
  executes and add some method to test for these conditions and be able
  to output some meaning full information.
  Eg,

     if (ctrl.execute())
     {
         if (!ctrl.always())
         {
             Info<< "Sampling executed based on " << ctrl.type() << nl;
         }
         ...
     }

     To produce "Sampling executed based on runTime"