Refactoring on Famix Simple Diff

[AntoninGoslin]

This PR mainly brings refactoring and cleaner code with her. Here is the list of the main ideas:

• Changed the printOn method. Each subclass defines how the printOn should be implemented; for example, additions only show what was added, deletions show what was deleted, etc.
• Renaming methods like FamixSimpleDifferenceEntityAddition into FSDEntityAddition.
• I added an exception message when the user tries to reach the list of methods when it is not created.
• Adding a new class to define the properties related to changes.
• Refactoring the way we are building the differences. Now every class has its method to build its own difference

[guillep]

Some small changes here and there!