Generate V_mimic for not triggered events

[abigailbishop]

If FILL_TREE_MODE is 0, all events are saved to the output AraSim file regardless of if they trigger or not. However, the Antenna_r.V_mimic objects (which mimic how our stations read out an event) are not generated unless an event triggers. This poses issues for near-threshold analyses such as the secondaries/multitriggers and cosmic ray simulation studies. It would be nice to allow AraSim to generate V_mimic objects for events regardless of triggering.