Augmenting276 add descriptions to symbols

should solve #276.

This adds descriptions to symbols (`Entity`) in the form of a `label` and a `comment` field.

Both are parsed by utilizing the logic-specific callback system and using the `text` attribute of an
axiom with an `AnnotationProperty` child.
This currently only works for **OWL** ontologies (thus the logic specific callback), since hets currently
does not seem to support more logics (with respect to descriptions).