mcmc convergence

The MCMC I've run on the [notebook](https://github.com/changhoonhahn/provabgs/blob/0010a3f47979d5e1be5003778dffff7ee166faa9/nb/tests/mcmc.ipynb) do not seem to be converged with either emcee or zeus. For zeus it seems to be particularly picky depending on the initialization. 

- [ ] _deploy on NERSC with very long iterations and examine its convergence properties_