Add trd hits to events

This MR takes care of the correct inclusion of TrdClusters and TrdHits into CbmEvents. Before only the digis were correcly linked in the events. Now, in case of an existing CbmEvent branch, the ClusterFinder and HitProducer only uses those digis linked in the events. The produced clusters and hits are again linked to the event.

@v.friese could you have a look please and let me know if I should change something or if the commits can be merged?

FYI @v.friese @d.smith During testing I made two observations independent of the changes @v.friese:

• EventBuilder "Real2019" seems to be quite unefficient in finding events based on our default transport/digi setttings. From 100 events only 13 are found. However, here I am not sure wether this expected or not. So just to have it mentioned.
• EventBuilder "Ideal" takes really long on 100 simulated events. Stage StsFindTracksEvents needs ~900 s per event. Again like above, might also expected.

Cheers, Pascal