function entropy( mu, beta ) {
	return addon( mu, beta );
}
Returns the differential entropy for a Gumbel distribution with location `mu` and scale `beta`.

@private
@param {number} mu - location parameter
@param {PositiveNumber} beta - scale parameter
@returns {number} entropy

@example
var y = entropy( 0.0, 1.0 );
// returns ~1.577

@example
var y = entropy( 5.0, 2.0 );
// returns ~2.27

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

@example
var y = entropy( 0.0, NaN );
// returns NaN

@example
var y = entropy( 0.0, 0.0 );
// returns NaN