simp_sat_cgain

  SIMP_SAT_CGAIN - Simple saturating contrast gain

   G = SIMP_SAT_CGAIN(C, Cs)

   Returns a simple contrast gain.  If contrast C is
   less than Cs, then G = 1.  Otherwise, gain is
   1-(C-Cs).  The gain falls off monotonically with
   slope 1 after the point Cs.