function median( lambda ) {
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return ceil( -ln( 0.5 ) / lambda ) - 1.0;
}
Returns the median of a Planck distribution.

@param {PositiveNumber} lambda - shape parameter
@returns {NonNegativeInteger} median

@example
var v = median( 0.1 );
// returns 6

@example
var v = median( 1.5 );
// returns 0

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

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