function mean( k ) {
	return addon( k );
}
Returns the expected value of a chi distribution.

@private
@param {number} k - degrees of freedom
@returns {number} expected value

@example
var y = mean( 2.0 );
// returns ~1.253

@example
var v = mean( 9.0 );
// returns ~2.918

@example
var v = mean( 1.0 );
// returns ~0.798

@example
var y = mean( NaN );
// returns NaN