cache this result, imports are a tad slow

We changed the logic to not be obsessed with NUTS. There is now a `failing` variable that the step methods report back, and that's passed to `update` to indicate sampling is failing. Also each step decides what is a meaningful stat to report. This reverts back to over-specializing for nuts

Any reason why these aren't methods anymore? It's not like they are used outside of ProgressBarManager (compute_draw_speed was static method anyway)