change `cpp_options[["stan_threads"]]` to `cpp_options[["STAN_THREADS"]]`

[toddpocuca]

This issue was discussed in wlandau/instantiate#30 and the solution recommended was to create an issue here to ask for capitalizing "stan_threads" in

cmdstanr/R/cpp_opts.R

Lines 149 to 171 in 5f2a8c8

	assert_valid_threads <- function(threads, cpp_options, multiple_chains = FALSE) {
	threads_arg <- if (multiple_chains) "threads_per_chain" else "threads"
	checkmate::assert_integerish(threads, .var.name = threads_arg,
	null.ok = TRUE, lower = 1, len = 1)
	if (is.null(cpp_options[["stan_threads"]]) || !isTRUE(cpp_options[["stan_threads"]])) {
	if (!is.null(threads)) {
	warning(
	"'", threads_arg, "' is set but the model was not compiled with ",
	"'cpp_options = list(stan_threads = TRUE)' ",
	"so '", threads_arg, "' will have no effect!",
	call. = FALSE
	)
	threads <- NULL
	}
	} else if (isTRUE(cpp_options[["stan_threads"]]) && is.null(threads)) {
	stop(
	"The model was compiled with 'cpp_options = list(stan_threads = TRUE)' ",
	"but '", threads_arg, "' was not set!",
	call. = FALSE
	)
	}
	invisible(threads)
	}

.

I don't know if this would be possible or if it would also be allowed for both "stan_threads " and "STAN_THREADS" to point to the same setting internally, but this would fix an issue with getting instantiate to allow for pre-compiling with multi-threading allowed without generating a warning.

[jgabry]

Thanks for opening the issue. If I recall correctly, I think this particular issue is being handled as part of #1022 (is that right @katrinabrock?), which is an overhaul of how we handle the C++ options.