Audio-only observations should default to 'animals'

Feature Requests
11

Merlin technically trains on the spectrogram images of the recordings rather than the audio itself. This works with the eBird/Macaulay Library audio database because a spectrogram is automatically generated for all uploaded audio. iNat currently doesn’t produce spectrogram images so CV training on them isn’t an option at this point, although it’s theoretically a possibility for the future.