Ontology featurization should support OMOP only

[EthanSteinberg]

Right now FEMR supports ontology expansion in both tabular and neural network featurization (https://github.com/som-shahlab/femr/blob/main/src/femr/featurizers/featurizers.py#L178 for tabular featurization and "hierarchical" for neural network featurization).

It would be useful to enforce additional variants here in some fashion.

In particular, we want to enforce a strict vocabulary here (OMOP only ideally).