tigefa4u projects Trac

* Attachment API simplified
* now uses a `TracObject` parent instead of `parent_type` and `parent_id` pairs
* function `attachment_to_hdf` is now the `Attachment.to_hdf` method
* unit tests changed accordingly
* Wiki and Ticket attachment code changed accordingly
* !TracObject refactorings:
* `XRef.XRefSystem` is now `TracObject.TracObjectSystem`
* `TracObject.factory` class method added
* inlined doc updates
* `WikiSystem._wiki_factory` and `TicketSystem._ticket_factory` methods added