Another approach to SeedingLayerSetNew

To keep SeedingLayer assignable
Add DetLayers to SeedingLayerSetNew
Add TrackingRegion::hits(const SeedingLayerSetNew::SeedingLayer&) interface
Add LayerHitMapCache::operator()(const SeedingLayerSetNew::SeedingLayer&, ...) interface
Fix TrackingRegion::hits interface, copy-paste implementation for new SeedingLayer
TrackingRegion knock-on on LayerHitMapCache
Add HitPairGeneratorFromLayerPairNew using the new SeedingLayerSet
Migrate CombinedHitPairGenerator to the new SeedingLayerSet
Add new version of LayerTriplets for SeedingLayerSetNew
Add HitPairGenerator::setSeedingLayers

Maybe this is needed in the future, maybe not, but I need it now to
continue playing around
Migrate HitTripletGeneratorFromPairAndLayers classes to SeedingLayerSetNew
Migrate CombinedHitTripletGenerator to SeedingLayerSetNew
Fix CombinedHitPairGenerator::clone()
Add CombinedHitPairGenerator::setSeedingLayers() (but leave unimplemented for now)
Migrate CombinedMultiHitGenerator, MultiHitGeneratorFromPairAndLayers, MultiHitGeneratorFromChi2
HitPairGeneratorFromLayerPairNew -> HitPairGeneratorFromLayerPair
Migrate configuration: TobTecLayerPairs_cfi
Migrate configuration: PixelLessLayerPairs_cfi
Migrate configuration: PixelLessLayerPairs4PixelLessTracking_cfi
Migrate configuration: PixelLayerTriplets_cfi