function gaussian( u ) {
	if ( isnan( u ) ) {
		return NaN;
	}
	return INV_SQRT_TWO_PI * exp( -0.5 * pow( u, 2.0 ) );
}
Computes the Gaussian kernel for a value.

@private
@param {number} u - input value
@returns {number} the value for the Gaussian kernel at u

@example
var u = 5.0;
var out = gaussian( u );
// returns ~0.0