adding cmdstanr as backend in shared params

[micahwiesner67]

We need to add the backend ("cmdstanr" or "rstan") in for the v2 release of epinow2pipeline

[micahwiesner67]

This is a pre-requisite for running the cfa-epinow2 v2 code. Feel free to approve whenever you have the opportunity and we can discuss when to pull this in

[natemcintosh]

So the epinow2 pipeline will read the new backend field, and pick the right one at runtime?

[micahwiesner67]

Yes, it will read it from this config. Technically we could run from either 'cmdstanr' or 'rstan' after the v2 release goes in, but from talking to Zach I believe we just want to support 'cmdstanr' going forward

[athowes]

Is this a breaking change if we were to merge it now? (Before the V2 release of the pipeline.)

[micahwiesner67]

@athowes It shouldn't be. This should be able to be added before v3 release of pipeline, we would just need to change the backend to 'rstan' to be accurate (this parameter wouldn't be being used, but for documentation sake should be accurate)

[athowes]

Yes my suggestion is that if this can go in in advance we might as well do that rather than have it sitting as an open PR!

[zsusswein]

I think this is a totally fine small change.

But! I think we should bump the config version because this is a breaking change for going forward. And perhaps we should think about allowing the pipeline to parse thing config_version field but that's probably overkill.

[micahwiesner67]

I created this branch just to demonstrate a basic way to test the 'rstan' backend. Can you check it out locally in VAP and just run 'make test-batch'. The following branch isn't meant to be pulled in, btw.

Canyo