Test created for within period check

Refactored viewWithin period, fix, datePair != null, it can never be
null. //redundant code.
Add displaying id in viewAll and viewPeriod
Refactored date related test to DatePairTest

First commit
Simplify DatePair.hasStartDate and DatePair.hasEndDate
Merge branch 'master' into quality_control
implement Delete
fix bug of test case for delete
change wrong return type for Delete()
Fix logic bug in DatePair.isWithinPeriod which happens when two dates are the same
Add tests for null dates in DatePair.isWithinPeriod
Merge branch 'quality_control'
implement updateTask() in excecute()
change update test case
delete the TODO comment in deleteTest
resolve markTaskCompleted TODO
change update() return comment
Merge remote-tracking branch 'origin/master' into update_logic
Merge branch 'master' into interface