function entropy( a, b ) {
	if (
		a >= b
	) {
		return NaN;
	}
	return ln( b-a );
}
Returns the differential entropy of a uniform distribution.

@param {number} a - minimum support
@param {number} b - maximum support
@returns {number} differential entropy

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

@example
var v = entropy( 4.0, 12.0 );
// returns ~2.079

@example
var v = entropy( -4.0, 4.0 );
// returns ~2.079

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

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

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