REC: improve success rate of merge of silicon and TPC

• Add fit direction option
  • default changed to forward, because of failure rate while backward
  • keep backward while merge group has only one track
• Change initial TrackState
  • Select source track according to TrackerHit number in track
  • forward: AtIP
  • backward: AtLastHit
  • While repeat after failure, use covMatrix as input
• If failure after try twice, copy old track from source of silicon tracks and TPC tracks