Add conditional load of newest classes/nodes.

To be able to have new GeoModel nodes/classes while not breaking the load of DBs generated with an earlier version of GeoModel, I added an 'optional' flag to the calls loading the new nodes from the DB. In that way, when loading a DB where we don't expect to see those new nodes, we have no errors as expected.

To achieve that, I also added the optional flag to the 'checkDBTable' and 'getPublishedNodes' methods.