Move find_features method into NucleotideSequence

This is about what I had in mind for #795.
I think we can entirely remove `find_features` and `feature_types` from `BiologicalSequence` since it would always raise a `ValueError`. If `BiologicalSequence` and/or `ProteinSequence` end up having type-specific features in the future we can always add the method back, but for now it's used only with `NucleotideSequence` and its subclasses.