This change allows you to set a subclass of
vtkm::cont::ExecutionObjectBase as a functor
used in ArrayHandleTransform. This latter class will then detect that
the functor is an ExecObject and will call PrepareForExecution with the
appropriate device to get the actual Functor object.

This change allows you to use virtual objects and other device dependent
objects as functors for ArrayHandleTransform without knowing a priori
what device the portal will be used on.