First steps to using OWLOntologyManagerFactory in PoddOWLManager

Once this abstraction is complete, users will not know where the
ontologies are stored in memory, as they cannot get access to an
OWLOntology object anymore to access this information.

This is necessary to easily abstract over an OWLDatabase, rather than an
in memory store.
Add PoddOWLManager.isCached method for testing
Remove single OWLOntologyManager from test
fix implementation of isCached to cope with versionless ontologies
Fix schema manager impl test that was relying on now redundant hack