Channel-partitioned TOF hits for tracking. Separate digi index output for cbm::tof::Hitfind.

The partitioning of the TOF hits as produced by the online TOF hitfinder was changed from RPC-wise to channel-wise, as requested by @s.zharko and @se.gorbunov.

Also, the indices of the contributing digis were removed from the cbm::online::algo::tof::Hit objects, as proposed by @fweig. This makes the hits much lighter objects and also produces a sizeable performance gain. The functionality of linking hits to digis is preserved, by storing all of the indices in a single vector, which is now part of the return-type of cbm::online::algo::tof::Hitfind (this is much faster than having one vector for each hit). Together with the number of contributing channels (which is now stored in the hit object), this vector can be used to obtain the same information as before. The class CbmTaskTofClusterizer was updated to support this.

A review from both @fweig and @s.zharko would be nice (@s.zharko can you comment on whether this is appropriate for tracking?).

Also of interest to @p.-a.loizeau @v.friese @i.deppner @n.herrmann