Sacdo: Fix std::pow(Fad,Const) again.

Created by: etphipp

My change of the formula for differentiating std:pow(x,y) with constant y seems to be causing some numerical stability issues (which is surprising because it just replaces x^y/x with x^{y-1}). So I am reverting that change and going back to the old formula. I still want to understand why it is problematic, but it is impacting a customer, so I am changing it now anyway. See here for the relevant discussion:

https://cee-gitlab.sandia.gov/Charon/tcad-charon/issues/96#note_179767