function lognormal( randn, mu, sigma ) {
	return exp( mu + (sigma*randn()) );
}
Returns a log-normally distributed pseudorandom number.

@private
@param {PRNG} randn - PRNG for standard normally distributed numbers
@param {number} mu - location parameter
@param {PositiveNumber} sigma - scale parameter
@returns {PositiveNumber} pseudorandom number