Refactoring

Refactoring done:

Refactor 1:
  Class: City.java
  	Method: makeTree()
  	Fix: Move the makeTree() method to another class called Tree.java, cause the method does not have any
  	     connection to City.java, and make City.java extend Tree.java, so that the HashSet and HashMap
  	     can be used by makeTree(), also the makeTree() method violets the Single responsibility principle.
  	Implementation: Implemented

Refactor 2:
  Class: City.java
    Fix: Remove the Compare(City c1, City c2) method, cause the CityComparator implements the same method and
         methodology. Just change the inner method methodology based on the compare method of City.java.
    Implementation: Implemented

Refactor 3:
  Class: City.java, RouteCost.java
    Fix: Remove the unused imported packages from the class, only add the needed ones.
    Implementation: Implemented

Refactor 4:
  Class: City.java, Link.java
  	Fix: Implement Encapsulation, encapsulate variables and provide getters and setters.
  	Implementation: Not Implemented


Extension provided:

  Class: RouteCost.java
  Extension Done: 1) Add color to the links(only "Red" and "Blue"), there can be now coloured and uncoloured links.
                     Handle the invalid input cases, error checking cases.
                  2) Input validation.