Trunk-4444 Validate order overlap with scheduled date

Comments:
In Order.java:
          - getCalculatedStartDate and getCalculatedEndDate are kept public for future use.
          - Can rename getCalculatedEndDate and getCalculatedStartDate with better names (if any).
U really wanted this method not to be on Order.java and instead just make a utility method in OrderServiceImpl so that you can ignore things like whether it is voided, same care setting etc. because it is going to be used in OrderServiceImpl and the code that calls it already has ensured these fields ate checked since it calls getActiveOrder