* Controller for budget scene in the application. This controller manages all actions that relates to the budget tableview (add, edit and delete), the switching
* Method that either reads data from a file with which it fills a budget register, if this is an old budget, or instantiates a budget register if this is a new budget.
* @param fileName The name of the file that is being read from.
* @return An object of type GeneralBudget.
* @throws IOException If an error occurs while reading from the file.