Linked Command and CommandExecutor.
refactored the logic package class. in specific:
1.in CommandExecutor, modified constructor, added accessor methods. refactored out the error message and put them in separate class called ErrorMessages
2. in logicfacade class, refactored enum to another separate class called Action. Also added getListMethod to return list to the GUI
3. added CommandType.java which contains the different command type . i.e: add, delete etc. This is to make it more OOP