CDF // 
FUNCTION: cdf( x, sigma )
Evaluates the cumulative distribution function (CDF) for a Rayleigh distribution with scale parameter `sigma` at a value `x`.

@param {Number} x - input value
@param {Number} sigma - scale parameter
@returns {Number} evaluated CDF
function cdf( x, sigma ) {
	if ( x < 0 ) {
		return 0;
	}
	var s2 = pow( sigma, 2 );
	return -expm1( - pow( x, 2) / ( 2 * s2 ) );
}