TimeValidity class. Coupled to Discount.
Stable version of TimeValidity. However it has low cohesion. A new
DateChecker class to do the checking jobs is recommended
TimeValidity stabilized.
Created TimeValidityDatesChecker.
TimeValidityDatesChecker took over the task of input controll.
Merge remote-tracking branch 'origin/master'
1.Discount and TimeValidity classes are simplified.
2.The tasks of checking the rules of a time validity are delegated to a
new class TimeValidityDatesChecker.
3.TimeValidity objects can now be compared upon the startDate
attribute's precedence.