Add explicit warning when number of dropouts is greater than number of enrolled

[Emmanuelludo]

rxsim/R/Population.R

Lines 165 to 167 in c4658a5

	# Don't drop more subjects than eligible (enrolled and not yet dropped)
	idx <- which(is.na(self$dropped) & !is.na(self$enrolled))
	n_use <- min(n, length(idx))