function entropy( a, b, c ) {
	return addon( a, b, c );
}
Returns the differential entropy of a triangular distribution.

@private
@param {number} a - minimum support
@param {number} b - maximum support
@param {number} c - mode
@returns {number} entropy

@example
var v = entropy( 0.0, 1.0, 0.5 );
// returns ~-0.193

@example
var v = entropy( 4.0, 12.0, 9.0 );
// returns ~1.886

@example
var v = entropy( -4.0, 4.0, -1.0 );
// returns ~1.886

@example
var v = entropy( 1.0, -0.1, 0.5 );
// returns NaN

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

@example
var v = entropy( NaN, 4.0, 2.0 );
// returns NaN

@example
var v = entropy( 0.0, NaN, 2.0 );
// returns NaN

@example
var v = entropy( 0.0, 4.0, NaN );
// returns NaN