improved branch stopper

• Only stop branch with >2 holes if it also has fewer than the required number of measurements. If there are enough measurements, we only stop it after 3 holes followed by another measurement. This logic prevents the branch being stopped (and therefore discarded) if there are missing measurements at the end of an otherwise perfectly good track.
• Branch stopper cut bins now use η from the current trackState, not from the original seed.
• Enable branch stopper by default (disable with flags.Acts.doTrackFindingTrackSelector=4).
• Rename maxHolesSelect → maxHoles → maxHolesBranch. Both maxHoles and maxHolesBranch are set from flags.Tracking.ActiveConfig.maxHoles, since they should do very nearly exactly the same thing.
• Simpler stopped branch message, now DEBUG.
• Refer to createITkTrackingPassFlags() in comment