Add date as a property to Item

src/main/java/no/ntnu/idatt1002/demo/data/Expense.java

src/main/java/no/ntnu/idatt1002/demo/data/Income.java

...main/java/no/ntnu/idatt1002/demo/data/IncomeCategory.java

src/main/java/no/ntnu/idatt1002/demo/data/Item.java

src/test/java/no/ntnu/idatt1002/demo/data/ExpenseTest.java

src/test/java/no/ntnu/idatt1002/demo/data/IncomeTest.java

src/test/java/no/ntnu/idatt1002/demo/data/ItemTest.java