Fixed error in predict_work_till_distance() function Added model_timing_gates_FD_fixed() and model_time_distance_FD_fixed() which allow user to set the fixed FD parameter. This was before done using additional parameter in model_timing_gates_FD() and model_time_distance_FD() functions, but now these functions only provide estimated FD. This is done to allow confidence intervals estimation, which was impossible to do since fixed FD was also estimated, but it was constrained by the optimization function to user-provided value.