Acts CKF track selector

• Add post-CKF track selector using Acts::TrackSelector. We can set default cuts absEtaMax=4, ptMin=0.4, and minMeasurements=7 to match Athena createITkTrackingPassFlags(), but Acts::TrackSelector doesn't allow η-dependent cuts (we use Athena's loosest cut for all η), nor selections on holes, shared hits, or separately on pixel/strip hits.
• Track selector can be disabled with flags.Acts.doTrackFindingTrackSelector=False. That just removes the specific cuts. This has a small overhead, so if this is a common requirement, we should disable the track selector (and post-CKF selection) altogether.
• finalize prints count of selected tracks (after output tracks, which is the total output from CKF, before the selection)
• Rename trackContainer to tracksContainer for consistency.