Improve the Newton start guess for very negative z in BR_z.
Improve the Newton start guess for very negative z in BR_z.

dftlibs/xcfun,robertodr/xcfun,robertodr/xcfun,dftlibs/xcfun,robertodr/xcfun,dftlibs/xcfun,bast/xcfun,bast/xcfun,dftlibs/xcfun,robertodr/xcfun,bast/xcfun,bast/xcfun