Modify LeaveEvent to accomodate periods of time, instead of single dates

Summary of outcomes for Data Standard 2.1

• Problem Addressed: The domain entity LeaveEvent can only accommodate one day of absence per LeaveEvent record.
• High-level Reasoning: Because the domain entity LeaveEvent only includes one date field, a staff leave event spanning multiple days must be entered with a unique record for each day of leave. This is inefficient and error-prone. Additionally, the model fails to differentiate between a staff absence event (missing all or some of one day of work) and a staff leave of absence (missing work over a period of time).
• Changes Made: Renamed domain entity LeaveEvent to StaffLeave; Changed EventDate to BeginDate and added optional EndDate; Renamed LeaveEventCategory to StaffLeaveEventCategory; Renamed LeaveEventReason to Reason; Removed HoursOnLeave field; Created StaffAbsenceEvent domain entity to accommodate absences that are not leaves of absence with identity consisting of EventDate, AbsenceEventCagory descriptor reference, and staff reference and including shared string AbsenceEventReason and shared decimal HoursAbsent as optional properties

Original ticket description follows:

The current LeaveEvent model has an EventDate as the only date element, and as a result an extended leave event - such as a maternity leave - is modeled as a series of largely repetitive LeaveEvents. This is inefficient and makes management more difficult and error-prone.

The task is to look at other models that resolve these problems.