function kurtosis( lambda ) {
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return 4.0 + ( 2.0 * cosh( lambda ) );
}
Returns the excess kurtosis of a Planck distribution.

@param {PositiveNumber} lambda - shape parameter
@returns {PositiveNumber} excess kurtosis

@example
var v = kurtosis( 0.1 );
// returns ~6.0100

@example
var v = kurtosis( 1.5 );
// returns ~8.7048

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

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