Common shared measurement computation?

in TrackFindingAlgorithm in the examples [(here)](https://github.com/acts-project/acts/blob/95e9c1e58b973a339bad3922555256a39255d607/Examples/Algorithms/TrackFinding/include/ActsExamples/TrackFinding/TrackFindingAlgorithm.hpp#L192) the `computeSharedHits` method is prefaced with the following comment:

```cpp
// TODO this is somewhat duplicated in AmbiguityResolutionAlgorithm.cpp
// TODO we should make a common implementation in the core at some point
template
void TrackFindingAlgorithm::computeSharedHits(
```
I think it can make sense to have this decoupled from the ambiguity solving algorithms -- I'm happy to work on this but I guess we should discuss.

@andiwand any thoughts (git-blame points to you for this comment)?

Another point to discuss -- we might want to add another entry to the `TrackStateFlag` enum, something like `MergedHitFlag`, which is a shared hit but that's identified as a "true" (or allowed) multi-particle state.