Add tests for ess and R-hat based on Stan's

[WardBrian]

Half of #69

Just using known values from Stan's tests

[magland]

Looks good to me. It's a shame to check in those large files, but as long as they don't change often I think it's fine.

[WardBrian]

Stan has been testing against these same files for 11 years, so I think we're probably fine there!

If we do touch this again, it will likely be to move it to another package entirely.

[jsoules]

It's a bit opaque, but seeing as it's imported directly from the Stan test suite and has been stable for over a decade, I'm okay to merge it.