function stdev( k ) {
	if ( isnan( k ) || k < 0.0 ) {
		return NaN;
	}
	return sqrt( 2.0 * k );
}
Returns the standard deviation of a chi-squared distribution.

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

@example
var v = stdev( 9.0 );
// returns ~4.243

@example
var v = stdev( 1.0 );
// returns ~1.414

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

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