function median( k, lambda ) {
	if (
		isnan( k ) ||
		isnan( lambda ) ||
		k <= 0.0 ||
		lambda <= 0.0
	) {
		return NaN;
	}
	return lambda * pow( LN2, 1.0/k );
}
Returns the median of a Weibull distribution.

@param {PositiveNumber} k - shape parameter
@param {PositiveNumber} lambda - scale parameter
@returns {PositiveNumber} median

@example
var v = median( 1.0, 1.0 );
// returns ~0.693

@example
var v = median( 4.0, 12.0 );
// returns ~10.949

@example
var v = median( 8.0, 2.0 );
// returns ~1.91

@example
var v = median( 1.0, -0.1 );
// returns NaN

@example
var v = median( -0.1, 1.0 );
// returns NaN

@example
var v = median( 2.0, NaN );
// returns NaN

@example
var v = median( NaN, 2.0 );
// returns NaN