double_exp

  DOUBLE_EXP - Double exponential

   G = DOUBLE_EXP(T, TAU1, TAU2)

     Returns ((TAU1*TAU2)/(TAU1-TAU2)) * (EXP(-T/TAU1)-EXP(-T/TAU2))

     Roughly speaking, if TAU1 and TAU2 are very different, then
     TAU1 is an onset time constant and TAU2 is an offset constant

     Time to peak is (TAU1*TAU2/(TAU1-TAU2))*LOG(TAU1/TAU2)