function pdf( x, mu, sigma ) {
	return addon( x, mu, sigma );
}
Evaluates the probability density function (PDF) for a normal distribution with mean `mu` and standard deviation `sigma` at a value `x`.

@private
@param {number} x - input value
@param {number} mu - mean
@param {NonNegativeNumber} sigma - standard deviation
@returns {number} evaluated probability density function

@example
var y = pdf( 2.0, 0.0, 1.0 );
// returns ~0.054

@example
var y = pdf( -1.0, 4.0, 2.0 );
// returns ~0.009

@example
var y = pdf( NaN, 0.0, 1.0 );
// returns NaN

@example
var y = pdf( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = pdf( 0.0, 0.0, NaN );
// returns NaN

@example
// Negative standard deviation:
var y = pdf( 2.0, 0.0, -1.0 );
// returns NaN

@example
var y = pdf( 2.0, 8.0, 0.0 );
// returns 0.0

@example
var y = pdf( 8.0, 8.0, 0.0 );
// returns Infinity