Datamodel to implement AbsenceNotice

AbsenceNotice is a new datastructure to define absence for a certain

• timeframe (day or multiple days)
• course, multiple courses or all courses
• certain lecture blocks of a specific course or all of the selected courses in that time frame

There are three types of absenceNotices: 

• Absence: student did not show up, nobody knows why
• Notified: student did not show up but called in a notified (e.g. sick)
• Dispense: medical certificate, funeral etc. 

AbsenceNotices can be entered before, during or while lecture blocks are running. Based on the AbsenceNotices the affected LectureBLockRollCalls will be filled out.