function upperGammaFraction( a, z ) {
	var f = upperIncompleteGammaFract( a, z );
	return 1.0 / ( z - a + 1.0 + continuedFraction( f ) );
}
Evaluate the lower incomplete gamma integral via a series expansion and divide by `gamma(z)` to normalize.

@private
@param {number} a - function parameter
@param {number} z - function parameter
@returns {number} function value