function rayleigh( rand, sigma ) {
	return sigma * sqrt( -2.0*ln( rand() ) );
}
Returns a pseudorandom number from a Rayleigh distribution with scale parameter `sigma`.

@private
@param {PRNG} rand - PRNG for generating uniformly distributed numbers
@param {PositiveNumber} sigma - scale parameter
@returns {NonNegativeNumber} pseudorandom number