Add EventDuration to CalendarDate

Nebraska uses the Calendar in the ODS to calculate the total number of days that a school is in session during the school year.  This is not limited to full days; a school or district could be in session for a partial day due to early dismissal or weather cancellation, etc. The EventDuration that was in the edfi.CalendarDateCalendarEvent that was in the 2.x data standard is no longer available.  We need some way of tracking partial days in the calendar.  Adding an EventDuration to the CalendarDate would allow us to make this calculation.