function stdev( lambda ) {
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return sqrt( exp( -lambda ) ) / abs( expm1( -lambda ) );
}
Returns the standard deviation of a Planck distribution.

@param {PositiveNumber} lambda - shape parameter
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 0.1 );
// returns ~9.9958

@example
var v = stdev( 1.5 );
// returns ~0.6080

@example
var v = stdev( -1.1 );
// returns NaN

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