Allow unigram tokenizer to handle NSNumber score values

[ZachNagengast]

For whatever reason, certain config files for unigram tokenizers end up reading scores as NSNumber instead of float, and throw malformedVocab errors.

Expected a Float but found __NSCFNumber

This doesn't convert directly to Float, so I'm adding one fallback here to attempt to convert it to float before throwing.

[pcuenca]

Interesting! Maybe because of precision issues?