extending GraphNode with generics

[Tommaso Teofili]

Hi all,
as discussed in CLEREZZA-537 it may be worth having GraphNodes use generics
to add something like "T extends Resource" parameter allowing easiest
subject type retrieving (avoiding useless casts, as said by Henry); I plan
to create a patch tomorrow so that anyone can review it and we can discuss
it (in a new issue).
Regards,
Tommaso