function stdev( k ) {
	var mu;
	if ( isnan( k ) || k < 0.0 ) {
		return NaN;
	}
	mu = mean( k );
	return sqrt( k - ( mu*mu ) );
}
Returns the standard deviation of a chi distribution.

@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