synonyms for trait classes

[stuckyb]

From a review comment on the PPO manuscript: "synonyms of the form "A B = B of A" should be added, where appropriate, to most terms in order to support better interoperability and further data exploitation via for instance NLP. E.g. "flower heads presence = presence of flower heads" (see how TO treats those cases)."

We agreed that this would be a good addition, and we need to figure out the best to do this and implement it.