function skewness( lambda ) {
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return 2.0 * cosh( lambda / 2.0 );
}
Returns the skewness of a Planck distribution.

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

@example
var v = skewness( 0.1 );
// returns ~2.0025

@example
var v = skewness( 1.5 );
// returns ~2.5894

@example
var v = skewness( -1.1 );
// returns NaN

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