Replace hybrid ranks with a hybrid_level property

[rynxs]

@upupa-epops

Once you choose one, the other(s) must be of the same rank.

Unfortunately this does not work well for plants, where some groups have infraspecific × species combinations, e.g. Asplenium × inexpectatum (A. rhizophyllum × ruta-muraria var. cryptolepis) or more complex nothospecies situations like Carex × flavicans, where nothovar. flavicans is C. aquatilis var. aquatilis × subspathacea and nothovar. substans is C. aquatilis var. minor × subspathacea.