function stdev( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	return sqrt( lambda );
}
Returns the standard deviation of a Poisson distribution.

@param {NonNegativeNumber} lambda - mean parameter
@returns {NonNegativeNumber} standard deviation

@example
var v = stdev( 9.0 );
// returns 3.0

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

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

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