function sigmoid (x, t) {
  x = NdArray.new(x).clone();
  t = t || 1;
  doSigmoid(x.selection, t);
  return x;
}
Return the sigmoid of the input array, element-wise.
@param {(Array|NdArray|number)} x
@param {number} [t=1] - stifness parameter
@returns {NdArray}