Add Optional CalendarReference to BellSchedule

It seems logical that a Bell Schedule would pertain to a particular Calendar.  That is how our SIS works, anyway.

It's nice that Ed-Fi 3.0 provides the flexibility to specify the exact dates on which a Bell Schedule is used.  However, I think we will often end up with the exact same set of dates on the Bell Schedule and its Calendar.  Putting an optional CalendarReference on the BellSchedule could eliminate the redundancy.