Create LogicMapping on Ontology import only if source and target have logics set. Refactored Hets module a little.
fix possible nil-value occurrence error

It seems possible that non_current_active_version?
can throw a Nil-Error, because ontology_version
can (but shouldn't) currently be nil.
make sure to erase previous set of categories before setting up new one
Merge branch 'staging' into augmenting119_categories_for_ontologies