Restructure subclassing in odml OWL file

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`.