function entropy( mu, sigma ) {
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	return 0.5 * ln( TWO_PI * E * sigma*sigma );
}
Returns the differential entropy for a normal distribution with mean `mu` and standard deviation `sigma`.

@param {number} mu - mean
@param {PositiveNumber} sigma - standard deviation
@returns {number} entropy

@example
var y = entropy( 0.0, 1.0 );
// returns ~1.419

@example
var y = entropy( 5.0, 3.0 );
// returns ~2.518

@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