Add 'depth' and 'provenance' to SynsetRelation

[DavidLeoni]

For fast retrieval of hypernyms / hyponyms at given depth adding a depth attribute to UBY SynsetRelation would help. To enable this, we can either

a) add a new table to the db (but this would require extra joins / duplicated data)
b) somewhat extend existing uby SynsetRelation class
c) use UBY's frequency attribute (but this alters its semantics)

Seems b) is possible by hacking a bit UBY hibernate configuration mechanism. We can:

1. create our own MySynsetRelation class which extends SynsetRelation
2. load all UBY hibernate hbm.xml files
3. replace in the files all the class strings referring to UBY SynsetRelation with our MySynsetRelation

Problem of the approach: the new attribute depth still won't be read from input LMF XMLs, but this is not an issue for now as depth can be computed and stored into the db each time the system is initialized.

[DavidLeoni]

Since it's not possible to have two SynsetRelations residing in separate lexicons and both starting from the same synset, we can add synthetic relations to the original lexicon and add a custom attribute provenance to mark the relation origin. To comply with Hibernate, it's also better to choose a one-table-per-hierarchy strategy and add a column DTYPE.