Add FairEventHeader to papa to get event start time
The start time of the event is needed for the calculation of beta from the time of flight. Up to now it was hard coded to 0 ns. With this change it is read from the event header instead.
The start time of the event is needed for the calculation of beta from the time of flight. Up to now it was hard coded to 0 ns. With this change it is read from the event header instead.