fix trigger dropping bug

- The trigger processing/writing function has an option to flush the
  trigger buffer after writing -- it is on by default
- Since triggers which occur after the file span still remained in the
  buffer, they were dropped mistakenly
    - This would only happen at file boundaries. It was noticed because
      the triggergen had been rewritten to output much larger buffers of
      triggers.
- The fix is to make the default flush=False
- Ideally, this should just be removed