switch to mirai for parallel processing?

It seems the latest version of `purrr` has enabled parallel processing using the `mirai` package, making it easier I think to do the ICI-Kt parallelized. https://purrr.tidyverse.org/dev/reference/parallelization.html

Would be worth checking if it makes it easier than the current switch between `purrr` and `furrr` we currently do.