Register (or override) an open-vocabulary Functional-sites key.
Parameters:
key (str) – The feature key (the user/predictor feature_type).
subcategory (str, optional) – Fine-grained label; defaults to 'FUNC_<key>'.
normalization (callable, optional) – Recipe applied to the raw per-residue values; defaults to a clip to
[0,1] (values must already lie in that range).