function entropy( mu, s ) {
	if (
		isnan( mu ) ||
		isnan( s ) ||
		s <= 0.0
	) {
		return NaN;
	}
	return ln( s ) + 2.0;
}
Returns the differential entropy for a logistic distribution with location `mu` and scale `s`.

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

@example
var y = entropy( 0.0, 1.0 );
// returns 2.0

@example
var y = entropy( 5.0, 2.0 );
// returns ~2.693

@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