More refactoring of LookupNames#add...subtaxa
More refactoring of LookupNames#add...subtaxa

- Refactor `add_immediate_subtaxa` for better performance (similar to `add_taxa`)
- Fix resulting ABC offenses by extracting methods

MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer