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

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

@example
var v = mode( 1.0, 1.0 );
// returns 0.0

@example
var v = mode( 4.0, 12.0 );
// returns ~11.167

@example
var v = mode( 8.0, 2.0 );
// returns ~1.967

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

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

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

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