Function russell_lab::sign

pub fn sign(x: f64) -> f64

Implements the sign function

          │ -1   if x < 0
sign(x) = ┤  0   if x = 0
          │  1   if x > 0

          |x|    x
sign(x) = ——— = ———
           x    |x|

sign(x) = 2 · heaviside(x) - 1

Reference: https://en.wikipedia.org/wiki/Sign_function