[CWEP002] properly handle serialization of computed relations We now: * have CWComputedRelation in the bootstrap schema to store computed relations * properly serialize/deserialize it * test first if the database has been migrated and contains the related table Related to #3546717 [jcr: adjust unittest_querier to pass with the added entity type]

raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")