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

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

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

@example
var v = entropy( 4.0, 12.0 );
// returns ~2.532

@example
var v = entropy( 8.0, 2.0 );
// returns ~0.119

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

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

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

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