Switch to taxrank for ranks, and decide on the metamodel

[cmungall]

Carried from

• #96.

Should we use taxrank to indicate ranks.

It's not clear if we can or should treat these as OMO/OIO subsets. This might be a time to switch to formal metaclasses:

• https://github.com/OBOFoundry/OBOFoundry.github.io/issues/2454