Consider moving the trigger_id and the timestamp outside the readout data structure

[lucabaldini]

It looks like we are passing the timestamp and keeping track of the trigger_id in all the different readout structures, while it would be more natural to let the (simulated) chip do its own things (sample the charge), and have another tiny module (e.g., bee.py) assigning these additional things, as in fact is happening with the HW.