database: add date filtering

- add methods to return deliveries, invoices and orders by date.
- takes in a two dates to serve as a range - returns items that fall within.
database: add unit tests

add unit tests for date range-selection
Merge branch 'date_ranges_db'

Conflicts:
	src/team4/retailsystem/model/PermanentDatabase.java
	test/team4/retailsystem/junit/PermanentDatabaseTest.java