Definition from ontologies

It would be great if the definition of an object is automatic set when an ontology reference is given.

We could test this functionality with the [G-Node OEN](https://github.com/G-Node/OEN) which should already contain the (one layer) sections and corresponding properties of the odML terminologies.

Example:
```python
ontology_url = "http://purl.org/incf/ontology/ExperimentalNeurophysiology/oen_term.owl/oen_0000442"
sec = odml.Section("MySubject", type="subject", reference=ontology_url) # or url to
sec.definition
# printout: "The investigated experimental subject (animal or person). May contain the Cell and Preparation sections as subsections."
```