Documentation on how to use save/load when using rbart_vi()

[jmobrien]

Small documentation note: the guidance for importing the trees into fitted object x is to call invisible(x$fit$state). However, in the case of rbart_vi(), x$fit is itself a list-object, requiring an adjustment to make the technique work properly (e.g., invisible(x$fit[[1]]$state) in the use case I encountered, where the list at x$fit contained a single element).

It would be great if the docs could be updated to help users be aware of this nuance. Thanks for all your work building this great tool.

EDIT: even this doesn't produce fully equivalent results when working with a loaded model vs one that was fit in the current session. I'm at a loss for how to proceed here.

(on Windows 10 using dbarts 0.9-22. This is all with educational data on a secure server, so I don't have a ready example to show, sorry.)