Event building improvement

[YifanC]

• 1. Move it to proto_nd_flow
• 2. Configure the readout window/ drift window to be v_drift (e_field) dependent
• 3. If it is a beam event, set the t0 to the middle of the beam window

4. Remove nhit_cut for external trigger (I think set up nhit_cut to 1 is clean enough to achieve the goal.)