Make container size for UT hits flexible

Implement a flexible container (SOACollection) for UT hits. Also figure out the maximum possible number of UT hits that can be written out in one event.

Adresses Moore#266 (closed) and specifically this line: https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/Event/TrackEvent/Event/PrUTHits.h#L26

Edited Apr 16, 2021 by Michel De Cian

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