Restructure subclassing in odml OWL file

[mpsonntag]

Currently odml/resources/odml-ontology.ttl contains the basic ontology for mapping odML entities to RDF classes. It also includes some example Section subclasses e.g. Cell or Hardware as proof of concept.

If we want to expand on the subclassing concept and add more terms on a regular basis it would probably be good to move the subclasses from odml-ontology.ttl to its own file e.g. odml-subclass-ontology.owl and import this file in odml-ontology.ttl.