Review RadiationTherapy feature

Does RadiationTherapy need a “performed at” similar to Procedure? Complicated because often multiple sessions are given.

Could deal with `RadiationTherapy` in a similar manner to the way we deal with medications. See https://phenopacket-schema.readthedocs.io/en/latest/treatment.html

Start by reading the treatment.html documentation linked in the issue, then inspect how RadiationTherapy is represented and how medications handle repeated or session-based treatment. Done means establishing whether a “performed at” field is appropriate, resolving the multiple-session concern, and updating the schema design accordingly.

Written by the indexing model from the issue text.