function mean( k, lambda ) {
	if (
		isnan( k ) ||
		isnan( lambda ) ||
		k <= 0.0 ||
		lambda <= 0.0
	) {
		return NaN;
	}
	return lambda * gamma( 1.0 + ( 1.0/k ) );
}
Returns the expected value of a Weibull distribution.

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

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

@example
var v = mean( 4.0, 12.0 );
// returns ~10.877

@example
var v = mean( 8.0, 2.0 );
// returns ~1.883

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

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

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

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