patch for complex conj of additions.

Dear GiNaCers, I found out that complex conjugating a-a*gamma5 gives a--a*gamma5. This is because the minus in front of a*gamm5 is stored inside the add while the minus that comes from cc-ing a*gamma5 is stored in the multiplication. These are not cancelled. The solution is to have a separate add::conjugate function. A patch is attached. Best, Chris