Add series for incomplete-gamma for when the fraction is slow.
o Add series for incomplete gamma function for small a and z. Needed because the continued fraction is slow in this range. o In INCOMPLETE-GAMMA-TAIL, call INCOMPLETE-GAMMA instead of CF-INCOMPLETE-GAMMA just in case a and z are small. o In INCOMPLETE-GAMMA, use the series for small a and z. o Simplify evaluation of Si(z) when z is real.