Consider supporting backwards compatiblity between ophys and pophys

[bruno-f-cruz]

A recent change made the previous tag ophys incompatible with the new pophys modality. In order to support previously serialized schemas, it would be nice to have some custom logic to coerce the previous definition to the current one.

This coercion mechanism should ideally make things compatible at the level of the python API (e.g. calling Modality.OPHYS) and at the level of deserialization. In both cases, the output should coerce to the new Modality.POPHYS object.