t0 reconstruction

Add a task CbmRecoTzero for reconstruction of the event t0. The current implementation just reads the time from the CbmTzdDigi in the event and fills the respective field in the CbmEvent class. In case there is no TzdDigi, the time is set to -1.; if there are more than one, the time is set to -2. These values are to be regarded as error codes.

The new task is added to the run_reco macro for the event-by-event case. If no TzdDigi branch is found, the task will be inactive.