Two-phase exponential decay function with time offset, x0 should be fixed.
Names: y0, x0, A1, t1, A2, t2
Meanings: y0 = offset, x0 = center, A1 = amplitude, t1 = time constant, A2 = amplitude, t2 = time constant
Lower Bounds: none
Upper Bounds: none
Note: different initial parameters are used for t1 and t2 so as to make sure t2 > t1 in the fitting results.