Add BFO roots for structural state and structural transition

[cthoyt]

This PR does two things:

1. Adds BFO:0000015 (process) as the parent of structural transition
2. Adds BFO:0000019 (quality) as the parent of structural state

because of #9, ontology tools (like the OLS) will still be able to guide users towards the right entry points when looking at the ontology

[XimeAixa]

We changed the main classes from structural state to state and from structural transition to transition. Would you like to update your PR to reflect this, or should I add the BFO roots?

[cthoyt]

I can take care of this!

[cthoyt]

I see you didn't merge the changes though, so I will have to wait for #14

[cthoyt]

@XimeAixa ready!