[Proposal] patient_id -> subject_id rename

[EthanSteinberg]

MEDS infrastructure is highly optimized for patient data, but there are some tooling that will probably work OK on non-patient data.

I would recommend we rename patient_id to subject_id to better support non-patient data.

Non-patient data is always going to be a second class citizen in the MEDS world, but this might make it less so.

[mmcdermott]

I would support this. I also think this is a good idea because not all medical data is patient data. E.g., eICU, the "patient" is actually a "patient in a single hospital admission" as you can't link reliably across hospital admissions.