0.6.0

Fixed a bug where the models of the transitive closure could not be saved.

The problem was that in some cases there is no resource for referenced models.
We solved this by saving the models n times (n = number of models in the transitive closure) until all models have a belonging resource. This is not the best way to do this. Better would be to calculate a dependency graph and save in an order that all referenced models are saved in advance. We choose to go the simpler way and introducing a bit of overhead for saving the models up to n times.