trajectory seeding with arbitrary seeding layers

Rewritten TrajectorySeedProducer to allow seeding using arbitrary/mixed numbers of seeding layers
https://indico.cern.ch/event/339411/session/0/contribution/5/material/slides/0.pdf

changes
- rewritten TrajectorySeedProducer
- renamed TrackerRecHit -> TrajectorySeedHitCandidate
- new class to hold the tree structure for seeding (SeedingTree)
- new class to check if a hit is on a requested layer (TrackingLayer)
- removed obsolete configurations
- updated configurations to new seeding syntax, old syntax no longer supported