cmdstanr unconstrain draws format changing

[n-kall]

There's a PR to change the output of unconstrain_draws to a draws object (stan-dev/cmdstanr/pull/886) so we can simplify the code below when that gets merged

iwmm/R/moment_match.R

Lines 601 to 608 in 38bdd3b

	udraws <- x$unconstrain_draws()
	udraws <- aperm(
	abind::abind(
	lapply(udraws, function(x) abind::abind(x, along = 2)),
	along = 3
	),
	perm = c(2, 3, 1)
	)