nRand lo hi n

Generates a single random float value in a sum of `n' uniform
distributions from `lo' to `hi'.

n = 1 : uniform distribution - same as Rand
n = 2 : triangular distribution
n = 3 : smooth hump
as n increases, distribution converges towards gaussian

> do { n <- nRand 1200.0 4000.0 (mce [2, 5])
>    ; let e = line KR 0.2 0 0.01 RemoveSynth
>      in audition (out 0 (fSinOsc AR n 0 * e)) }