function mean( a, b, c ) {
	return addon( a, b, c );
}
Returns the expected value (mean) of a triangular distribution.

@private
@param {NonNegativeNumber} a - minimum support
@param {NonNegativeNumber} b - maximum support
@param {NonNegativeNumber} c - mode
@returns {NonNegativeNumber} expected value (mean)

@example
var v = mean( 0.0, 10.0, 5.0 );
// returns 5.0

@example
var v = mean( 2.0, 8.0, 4.0 );
// returns ~4.667

@example
var v = mean( -1.0, 5.0, 6.0 );
// returns NaN

@example
var v = mean( NaN, 10.0, 5.0 );
// returns NaN