function skewness( lambda ) {
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return 1.0 / sqrt( lambda );
}
Returns the skewness of a Poisson distribution.

@param {PositiveNumber} lambda - mean parameter
@returns {PositiveNumber} skewness

@example
var v = skewness( 9.0 );
// returns ~0.33

@example
var v = skewness( 1.0 );
// returns 1.0

@example
var v = skewness( -0.2 );
// returns NaN

@example
var v = skewness( NaN );
// returns NaN