Added FileHandling class for ItemRegister objects, and reworked ItemRegister with new subclasses

ItemRegister now has two subclasses, one for storing Income (IncomeRegister) and one for storing Expense (ExpenseRegister), and I added tests for these classes.

FileHandling is able to write an IncomeRegister and ExpenseRegister to and from a file.

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

@@ -58,4 +58,14 @@ public class Expense extends Item{
         }
         this.category = expenseCategory;
     }
+
+    /**
+     * Returns a String that represents the Expense.
+     * Also includes the ExpenseCategory
+     * @return The Expense as a String
+     */
+    @Override
+    public String toString() {
+        return super.toString()+"\ncategory="+category.toString()+"\n";
+    }
 }