Using stan_jm for parametric proportional hazards regression only

@kaybenleroll and @ermeel or anyone else interested, can you start a separate topic for this? We can discuss the best way to collaborate on this and whether it should be part of an existing package or a new one.

@Gianluca_Baio, Thanks for the update. I’ll definitely take a closer look at survHE! I looked enough that I felt comfortable recommending it here but I’m not super familiar with it yet.