Association of Issues to Nodes

[ftomassetti]

At this time Issues are associated to positions. I wonder if alternatively they could be associated to Nodes. In this way we could use them also when we have ASTs which are built programmatically. We may also want to associate them with entire files. It is true that we could associate them with the root of the AST, but in case we cannot build the root because of errors, it may be useful to have the possibility to represent "global" errors. Finally Issues may be also associate to entire projects perhaps