function entropy( k ) {
	return addon( k );
}
Returns the differential entropy of a chi distribution.

@private
@param {PositiveNumber} k - degrees of freedom
@returns {number} entropy

@example
var v = entropy( 9.0 );
// returns ~1.052

@example
var v = entropy( 1.0 );
// returns ~0.726

@example
var v = entropy( -0.2 );
// returns NaN

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