nSharedHits in trajectory not working

[ShujieL]

Environment: (where does this bug occur, have you tried other environments)

• Which branch (often main for latest released): main
• Which version (or HEAD for the most recent on git): HEAD
• Any specific OS or system where the issue occurs? default container
• Any special versions of ROOT or Geant4? N/A

Steps to reproduce: (give a step by step account of how to trigger the bug)

Minjung Kim noticed that in any EICrecon output, the trajectory.nSharedHits is always 0, though the trajectories are obviously duplicated. see below:

 CentralCKFSeededTrajectories.type = 0, 0, 0
 CentralCKFSeededTrajectories.nStates = 15, 15, 15
 CentralCKFSeededTrajectories.nMeasurements = 6, 6, 6
 CentralCKFSeededTrajectories.nOutliers = 0, 0, 0
 CentralCKFSeededTrajectories.nHoles = 1, 1, 1
 CentralCKFSeededTrajectories.chi2 = 5.717027, 5.521172, 5.345023
 CentralCKFSeededTrajectories.ndf = 12, 12, 12
 CentralCKFSeededTrajectories.nSharedHits = 0, 0, 0

Expected Result: (what do you expect when you execute the steps above)

The nshared hits should have the value set in CKFtracking.cc: https://github.com/eic/EICrecon/blob/f8e335686455506ee10e537cb5c5b8b1d15bf677/src/algorithms/tracking/CKFTracking.cc#L238

[ShujieL]

The MultiTrajectoryHelper identify a shared hit by "SharedHitFlag". But CKF algorithm (CombinatorialKalmanFilter.hpp) never set this flag, that's why EICrecon always pick up 0. We can either add a piece of code in MultitrajectoryHelper (ACTS) or CKFtracking (EICrecon) that loops over hits to find shared hits . @wdconinc any thoughts?