function kurtosis( k, lambda ) {
	return addon( k, lambda );
}
Returns the excess kurtosis of an Erlang distribution.

@private
@param {PositiveInteger} k - shape parameter
@param {PositiveNumber} lambda - rate parameter
@returns {NonNegativeNumber} excess kurtosis

@example
var v = kurtosis( 1, 1.0 );
// returns 6.0

@example
var v = kurtosis( 4, 12.0 );
// returns 1.5

@example
var v = kurtosis( 8, 2.0 );
// returns 0.75

@example
var v = kurtosis( 1, -0.1 );
// returns NaN

@example
var v = kurtosis( -0.1, 1.0 );
// returns NaN

@example
var v = kurtosis( 2, NaN );
// returns NaN

@example
var v = kurtosis( NaN, 2.0 );
// returns NaN