Adding an option to Coulomb.cc to allow the use of sigmoidal dielectric in fa_elec/fa_grpelec.
It is turned on with the flag -elec_sigmoidal_die, and there are three parameters that control its behavior:
-elec_sigmoidal_die_D0 : the dielectric at short range
-elec_sigmoidal_die_D : the dielectric at long range
-elec_sigmoidal_die_S : the transition between the two (higher is faster fade)