There was a bug in "get ArrayList" functions, now it is fixed.

Link/unlink function in Port has been revised.

CompositeEntity now contains a Relation collection (initially not
provided, but it is correct that a Relation is contained somewhere),
with the related functions (add, get, etc.) and two methods to
connect/disconnect ports quickly, but major improvements are needed (the
most important is to create a new Relation with an unique name, without
explicity providing).

Relation provides just LinkedPortList() function, add/remove/get Port
temporary commented, maybe they will be eliminated soon.

Test contains a simple composition and a visit function (BFS), it seems
to work!