Rename `netket.stats.Stats.R` to `Rhat`?

Created by: femtobit

As we have seen in the discussion on #365, the stats.R parameter is ambiguously named; it is sometimes called "Gelman-Rubin" statistic or "potential scale reduction", but I think the most common way to refer to it is \hat R in LaTeX math or "Rhat" in words (this name is used, e.g., by the stan framework). So I propose we also call it Rhat starting in 3.0.

What do you think?