Improve the apomorphy description in apomorphy-based phylorefs

[gaurav]

In PR #291, we implement apomorphy fields exactly as specified in the Phyx manuscript -- with a definition, bearingEntity and phenotypicQuality field. As per #291 (comment), it might make sense to replace that with just the free-text definition for now. If so, we would need to decided whether we want to make that change in phyx.js, or leave that functionality available but ignore it in Klados. I'm inclined to leave it as-is for Klados 1.0, and then improve it later.

In the future, we might want a "related entity" field as well. Gaurav to talk to Jim about this. Maybe a Manchester syntax field?