ActsTrackReconstruction: Refactor of TrackFindingAlg (part 1 of N)

This MR is an attempt to refactor TrackFindingAlg class, so it can be later split into the base and the derived class. The ultimate goal is to merge TrackFindingAlg and TrackExtensionAlg together (part 1 of N).

What was done:

• New method to avoid code duplication when merging Seed/UncalibratedMeasurement/SiDetectorElements collections together (getContainersFromKeys).
• Moved code around measurement selector into a separate method: setMeasurementSelector() (common code with TrackExtensionAlg).

Tagging @tbold @goetz for discussion.