function stdev( a, b, c ) {
	return addon( a, b, c );
}
Returns the standard deviation of a triangular distribution.

@private
@param {number} a - minimum support
@param {number} b - maximum support
@param {number} c - mode
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 0.0, 1.0, 0.5 );
// returns ~0.204

@example
var v = stdev( 4.0, 12.0, 9.0 );
// returns ~1.65

@example
var v = stdev( -4.0, 4.0, -1.0 );
// returns ~1.65

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

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

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

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

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