Windows 10 parallelization can be super slow

[avehtari]

Lawrence Hunsicker reported that Windows 10 is super slow in parallelization if .rprofile has options(mc.cores = parallel::detectCores()). The same line in a script is not a problem,
https://discourse.mc-stan.org/t/problem-parallelizing-rstanarm-loo/6433/10

I don't know what we can do for this in loo package, but I add this here if someone is hit by the same Windows bug.

[jgabry]

We could mention this in the package documentation, but if this is very severe then maybe it's worth printing a message about it to Windows users when the package is loaded?
I'm not sure what the message would be precisely, but it's possible by adding to the .onAttach() function. Currently it's this:

loo/R/zzz.R

Lines 1 to 10 in e147fd3

	.onAttach <- function(...) {
	ver <- utils::packageVersion("loo")
	packageStartupMessage(
	"This is loo version ", ver, ".\n",
	"**NOTE: As of version 2.0.0 loo defaults to 1 core ",
	"but we recommend using as many as possible. ",
	"Use the 'cores' argument or set options(mc.cores = NUM_CORES) for an entire session. ",
	"Visit mc-stan.org/loo/news for details on other changes."
	)
	}