Create Income and Expense as subclasses of Item with respective category enum classes

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

...ain/java/no/ntnu/idatt1002/demo/data/ExpenseCategory.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/ItemTest.java