Fix error handling of BaseEvaluator When restructuring the evaluation, I forgot to put the rescue block cancelling the concurrency handling into the ConcurrentEvaluator.

Rename attribute hets_evaluator to importer.

Move concurrency handling to a separate class The new ConcurrentEvaluator is only used for Ontology parsing. It has all the functionality that the BaseEvaluator had before. The new BaseEvaluator, however, is the base class for the ProveEvaluator and does not contain any concurrency handling which renders the concurrency reladed methods in the ProveEvaluator needless.

Allow to overwrite BaseEvaluator's methods.

Move Ontology parser to DG module A Prove module will be added for parsing of the prove-command output.