Documentation about output formats

[steven-murray]

In general, it would be nice if all theory predictions from HERA were output in a uniform format, so that whoever is writing the paper at the end can more easily compare them. The ultimate way to deal with this might be to have a separate repo in which we have scripts that convert between different output formats, or at least between different output formats and a single most general format that we want.

In the meantime, it may be useful to think about what kind of format we might like to have, and provide documentation in this repo that guides theorists in this regard.

Some useful packages that implement data-formats for MCMC chains:

• https://arviz-devs.github.io/arviz/index.html
• https://getdist.readthedocs.io/en/latest/

I don't much like getdist, but it is the default output format for multinest/polychord. We could consider writing a small plugin for arviz that will convert this datatype to its native xarray format. arviz provides conversion functions for many formats already (including emcee and pyMC3).