Additional special cases in the handling of pow(Jet,Jet).

I think this is all of the cases. These cases arise because pow(a,b) is limited
to real valued results, if the argument and result were complex valued then
these cases would disappear.

NOTE: Since there is so much special casing here, it is worth checking to see
if cpow() is implemented in terms of pow(), and what might be the consequences
of using cpow() on the type std::complex<Jet<double, N> >. It is *possible*
that a separate implementation of cpow might be required also.

Also some comment fixes.

Change-Id: Ia1e38df4cdcb548f778304c2854cacba6e1556ff