ACTS TF: compute shared hits

Separates out measurementContainerOffsets() and measurementOffsets() from ActsTrk::detail::TrackFindingMeasurements into a new class ActsTrk::detail::SharedHitCounter.

• This class needs the hits provided with its own addMeasurements()
• It provides a method computeSharedHits() to record shared hits in a newly created track. This is used by TrackFindingAlg and TrackExtensionAlg to add shared hit flags to their output tracks.
• As a by-product, also provides method measurementIndex(hit), which is a unique hit index that is printed by TrackStatePrinter.