The result, say
r <- x + runif(n, -a, a)
n <- length(x) and
a is the
argument (if specified).
z <- max(x) - min(x) (assuming the usual case).
a to be added is either provided as positive
amount or otherwise computed from
amount == 0, we set
a <- factor * z/50 (same as S).
NULL (default), we set
a <- factor * d/5 where d is the smallest
difference between adjacent unique (apart from fuzz)