Functors for the smog minimum bias line
Following the presentation held at the 93rd LHCb week, this merge request is part of the effort put in place for the Smog2 minimum bias line implementation. This essentially requires:
- the z coordinate of track first state in the Smog2 region.
- a high track number of hits.
Functors for both requests have to be implemented. This MR defines the nHits accessor required for the NHITS functor implementation, see also the corresponding Rec (Rec!1703 (merged)) and Moore MRs.