function stdev( k ) {
	return addon( k );
}
Returns the standard deviation of a chi distribution.

@private
@param {NonNegativeNumber} k - degrees of freedom
@returns {NonNegativeNumber} standard deviation

@example
var v = stdev( 9.0 );
// returns ~0.697

@example
var v = stdev( 1.0 );
// returns ~0.603

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

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