function kurtosis( k ) {
	return addon( k );
}
Returns the excess kurtosis of a chi-squared distribution.

@private
@param {PositiveNumber} k - degrees of freedom
@returns {number} excess kurtosis

@example
var v = kurtosis( 9.0 );
// returns ~1.333

@example
var v = kurtosis( 1.0 );
// returns 12.0

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

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