SBC for arma model

@martinmodrak This is a good point. Most of the diagnostics are unavailable as I’m running everything on a single-chain basis. I had assumed since the benchmark models have been extensively tested that there shouldn’t be any issues, but that only applies for the given datasets.

I’ve also since realised that it isn’t necessary to do SBC with single independent chains, so my plan is to do a multiple-chain implementation and then I can look at things like Rhat. I can then hopefully look for non-convergence and other pathologies.