Add multiprocess, which is multicore with multisession as a fallback

[HenrikBengtsson]

Add multiprocess, which is multicore with multisession as a fallback, such that one can suggest this regardless of platform, i.e.

plan(multiprocess)

Implementation would be something like:

multiprocess <- function(...) {
  fcn <- if (supportsMulticore()) multicore else multisession
  fcn(...)
}