Fix a bug in vtkExodusIIReader when # of timesteps equals one.
When reading Exodus files, if there are zero or one timesteps, then there is only one file containing the data to be read. So, we treat both instances in the same manner.
When reading Exodus files, if there are zero or one timesteps, then there is only one file containing the data to be read. So, we treat both instances in the same manner.